Precise P-x data have been measured for the binary systems 1-hexene/3-methyl cyclopentene at 75 degrees C, 1-pentene/2-methyl butane, 1-pentene/N-methyl pyrrolidone-2 (NMP), 2-methyl butane/NMP, n-hexane/NMP, 2-methyl pentane/NMP, 3-methyl pentane/NMP, 1-hexene/NMP, 3-methyl cyclopentene/NMP at 90 degrees C and 1-hexene/NMP at 140 degrees C using a static apparatus. Activity coefficients at infinite dilution have been obtained from P - x data in the dilute region and additionally measured at 70 and 80 degrees C for n-hexane, 2-methyl pentane, 3-methyl pentane, 1-hexene and 3-methyl cyclopentene in NMP using gas-liquid chromatography. Furthermore for all the above mentioned systems heats of mixing have been determined with the help of an isothermal flow calorimeter. NRTL parameters have been fitted simultaneously to all the experimental data.