Precise P-x data have been measured for the binary system water+N-methyl-2-pyrrolidone (NMP) at (80 and 110)degrees C using a static apparatus. Furthermore, enthalpies of mixing have been determined at (90, 140, and 143)degrees C with an isothermal flow calorimeter. Temperature dependent NRTL parameters have been fitted simultaneously to all the experimental data. The experimental data have been compared with the predicted results of the modified UNIFAC method, for which a new flexible lactam (cyclic amide) group was defined.