P rho T properties in the compressed liquid phase were measured for the system N-methyldiethanolamine (MDEA) (1) + water (2) at temperatures between (313 and 363) K and pressures up to 20 MPa. Densities of MDEA and four gravimetrically prepared mixtures of MDEA (1) + water (2), at x(1) = 0.0369, 0.0607, 0.0893, and 0.1302, were determined using a vibrating tube densimeter. The classical calibration method of the vibrating tube densimeter was used, using nitrogen and water as reference fluids. The uncertainty was estimated to be +/- 0.2 kg.m(-3) for the measured densities. The densities of each mixture and of MDEA were correlated using a volume explicit equation of six parameters. Isothermal compressibilities and isobaric thermal expansivities were calculated using the six-parameter equation with the correlated parameters obtained for MDEA and for the four mixtures. The uncertainties on these properties were estimated to be +/- 0.005 GPa(-1) and +/- 5.10(-7) K-1, respectively. Excess molar volumes for the mixtures were determined using the measured densities of the mixture and MDEA volumes calculated from the six-parameter equation and water volumes calculated from a multiparameter reference equation of state (EoS). The uncertainty in excess molar volumes was estimated to be +/- 0.006 cm(3).mol(-1).