diff --git a/src/musredit_qt5/mupp/PmuppGui.cpp b/src/musredit_qt5/mupp/PmuppGui.cpp
index fa067f25..d627c91e 100644
--- a/src/musredit_qt5/mupp/PmuppGui.cpp
+++ b/src/musredit_qt5/mupp/PmuppGui.cpp
@@ -205,6 +205,7 @@ PmuppGui::PmuppGui( QStringList fln, QWidget *parent, Qt::WindowFlags f )
 {
   QDateTime dt = QDateTime::currentDateTime();
   fDatime = dt.toTime_t();
+  fMuppInstance = -1;
 
   fMuppPlot = 0;
 
@@ -414,8 +415,12 @@ void PmuppGui::aboutToQuit()
 
   // clean up temporary plot files
   QProcessEnvironment env = QProcessEnvironment::systemEnvironment();
-  QString pathName = QString("%1/.musrfit/mupp/_mupp_%2.dat").arg(env.value("HOME")).arg(fDatime);
-  pathName = QString("%1/.musrfit/mupp/_mupp_ftok.dat").arg(env.value("HOME"));
+  if (fMuppInstance != -1) {
+    QString pathName = QString("%1/.musrfit/mupp/_mupp_%2.dat").arg(env.value("HOME")).arg(fDatime);
+    QFile::remove(pathName);
+    pathName = QString("%1/.musrfit/mupp/_mupp_ftok_%2.dat").arg(env.value("HOME")).arg(fMuppInstance);
+    QFile::remove(pathName);
+  }
 
   // needed for clean up and to save the cmd history
   writeCmdHistory();
@@ -584,7 +589,7 @@ void PmuppGui::fileOpen()
  */
 void PmuppGui::fileExit()
 {
-  qApp->quit();
+  aboutToQuit();
 }
 
 //----------------------------------------------------------------------------------------------------
@@ -1616,13 +1621,16 @@ void PmuppGui::plot()
 
   file.close();
 
+  // get first free mupp instance
+  fMuppInstance = getFirstAvailableMuppInstance();
+
   // issue a system message to inform to ROOT parameter plotter (rpp) that new data are available
   key_t key;
   struct mbuf msg;
   int flags, msqid;
 
   // generate the ICP message queue key
-  QString tmpPathName = QString("%1/.musrfit/mupp/_mupp_ftok.dat").arg(env.value("HOME"));
+  QString tmpPathName = QString("%1/.musrfit/mupp/_mupp_ftok_%2.dat").arg(env.value("HOME")).arg(fMuppInstance);
   file.setFileName(tmpPathName);
   if (!file.open(QIODevice::WriteOnly | QIODevice::Text)) {
     QMessageBox::critical(this, "ERROR", "Couldn't write necessary temporary file!");
@@ -1632,7 +1640,7 @@ void PmuppGui::plot()
   fout << QCoreApplication::applicationFilePath().toLatin1().data() << endl;
   file.close();
 
-  key = ftok(QCoreApplication::applicationFilePath().toLatin1().data(), 1);
+  key = ftok(QCoreApplication::applicationFilePath().toLatin1().data(), fMuppInstance);
   if (key == -1) {
     QMessageBox::critical(this, "ERROR", "Couldn't obtain necessary key to install the IPC message queue.");
     return;
@@ -1677,11 +1685,14 @@ void PmuppGui::startMuppPlot()
   QProcessEnvironment env = QProcessEnvironment::systemEnvironment();
   QString cmd = QString("%1/bin/mupp_plot").arg(MUPP_PREFIX);
 #if defined(Q_OS_DARWIN) || defined(Q_OS_MAC)
-  cmd = QString("/Applications/mupp.app/Contents/MacOS/mupp_plot"); // as35 not yet ready
+  cmd = QString("/Applications/mupp.app/Contents/MacOS/mupp_plot");
 #endif
   QString workDir = QString("./");
   QStringList arg;
 
+  // feed the mupp instance
+  arg << QString("%1").arg(fMuppInstance);
+
   fMuppPlot = new QProcess(this);
   if (fMuppPlot == nullptr) {
     QMessageBox::critical(0, "**ERROR**", "Couldn't invoke QProcess for mupp_plot!");
@@ -1996,3 +2007,27 @@ void PmuppGui::selectCollection(QString cmd)
     }
   }
 }
+
+//----------------------------------------------------------------------------------------------------
+/**
+ * @brief PmuppGui::getFirstAvailableMuppInstance
+ * @return
+ */
+uint PmuppGui::getFirstAvailableMuppInstance()
+{
+  // if fMuppInstance already set, i.e. != -1, do nothing
+  if (fMuppInstance != -1)
+    return fMuppInstance;
+
+  QProcessEnvironment env = QProcessEnvironment::systemEnvironment();
+  QString fln("");
+  uint i=0;
+  for (i=0; i<256; i++) {
+    fln = QString("%1/.musrfit/mupp/_mupp_ftok_%2.dat").arg(env.value("HOME")).arg(i);
+    if (!QFile::exists(fln))
+      break;
+  }
+
+  return i;
+}
+
diff --git a/src/musredit_qt5/mupp/PmuppGui.h b/src/musredit_qt5/mupp/PmuppGui.h
index 366fed22..4056b33d 100644
--- a/src/musredit_qt5/mupp/PmuppGui.h
+++ b/src/musredit_qt5/mupp/PmuppGui.h
@@ -129,6 +129,7 @@ private:
   bool fDarkToolBarIcon;
 
   uint fDatime;
+  uint fMuppInstance;
 
   PParamDataHandler *fParamDataHandler;
   QVector<PmuppXY> fXY;
@@ -181,6 +182,7 @@ private:
   void getMinMax(QVector<double> &data, double &min, double &max);
   QString substituteDefaultLabels(QString label);
   void selectCollection(QString cmd);
+  uint getFirstAvailableMuppInstance();
 
 private slots:
   void refresh();
diff --git a/src/musredit_qt5/mupp/plotter/PMuppCanvas.cpp b/src/musredit_qt5/mupp/plotter/PMuppCanvas.cpp
index 5c20843c..4d483bba 100644
--- a/src/musredit_qt5/mupp/plotter/PMuppCanvas.cpp
+++ b/src/musredit_qt5/mupp/plotter/PMuppCanvas.cpp
@@ -88,7 +88,9 @@ PMuppCanvas::PMuppCanvas()
  */
 PMuppCanvas::PMuppCanvas(const Char_t *title, Int_t wtopx, Int_t wtopy,
                          Int_t ww, Int_t wh, const PIntVector markerSytleList,
-                         const PDoubleVector markerSizeList, const PIntVector colorList) :
+                         const PDoubleVector markerSizeList, const PIntVector colorList,
+                         const int mupp_instance) :
+  fMuppInstance(mupp_instance),
   fMarkerStyleList(markerSytleList),
   fMarkerSizeList(markerSizeList),
   fColorList(colorList)
@@ -303,7 +305,8 @@ void PMuppCanvas::CheckIPCMsgQueue()
       cerr << "**ERROR** couldn't get value of the environment variable HOME." << endl << endl;
       return;
     }
-    strncat(str, "/.musrfit/mupp/_mupp_ftok.dat", sizeof(str)-1);
+    memset(str, '\0', sizeof(str));
+    snprintf(str, sizeof(str), "%s/.musrfit/mupp/_mupp_ftok_%d.dat", getenv("HOME"), fMuppInstance);
     ifstream fin(str, ifstream::in);
     if (!fin.is_open()) {
       cerr << endl;
@@ -315,7 +318,7 @@ void PMuppCanvas::CheckIPCMsgQueue()
     fFtokName = str;
   }
 
-  key = ftok(fFtokName.Data(), 1);
+  key = ftok(fFtokName.Data(), fMuppInstance);
   flags = IPC_CREAT;
   msqid = msgget(key, flags | S_IRUSR | S_IWUSR);
   if (msqid == -1) {
diff --git a/src/musredit_qt5/mupp/plotter/PMuppCanvas.h b/src/musredit_qt5/mupp/plotter/PMuppCanvas.h
index 62678489..16d15214 100644
--- a/src/musredit_qt5/mupp/plotter/PMuppCanvas.h
+++ b/src/musredit_qt5/mupp/plotter/PMuppCanvas.h
@@ -74,7 +74,8 @@ public:
   PMuppCanvas(const Char_t* title,
               Int_t wtopx, Int_t wtopy, Int_t ww, Int_t wh,
               const PIntVector markerSytleList, const PDoubleVector markerSizeList,
-              const PIntVector colorList);
+              const PIntVector colorList,
+              const int mupp_instance);
   virtual ~PMuppCanvas();
 
   virtual Bool_t IsValid() { return fValid; }
@@ -89,6 +90,7 @@ public:
 
 private:
   Bool_t fValid;
+  Int_t fMuppInstance;
 
   TString fFtokName;
   TTimer *fCheckMsgQueue; ///< timer needed to check if a message in the IPC message queue is pending
diff --git a/src/musredit_qt5/mupp/plotter/mupp_plot.cpp b/src/musredit_qt5/mupp/plotter/mupp_plot.cpp
index efc7d817..6affb077 100644
--- a/src/musredit_qt5/mupp/plotter/mupp_plot.cpp
+++ b/src/musredit_qt5/mupp/plotter/mupp_plot.cpp
@@ -46,6 +46,14 @@ using namespace std;
  */
 int main(int argc, char *argv[])
 {
+  if (argc != 2) {
+    return -1;
+  }
+  int mupp_instance = (int)strtol(argv[1], NULL, 10);
+  if ((mupp_instance < 0) || (mupp_instance > 255)) {
+    return -2;
+  }
+
   // read startup file
   char startup_path_name[128];
   TSAXParser *saxParser = new TSAXParser();
@@ -93,7 +101,8 @@ int main(int argc, char *argv[])
   PMuppCanvas *muppCanvas = new PMuppCanvas("mupp", 10, 10, 600, 800,
                                             startupHandler->GetMarkerStyleList(),
                                             startupHandler->GetMarkerSizeList(),
-                                            startupHandler->GetColorList());
+                                            startupHandler->GetColorList(),
+                                            mupp_instance);
 
   if (muppCanvas != 0) {
     if (muppCanvas->IsValid()) {