Apparent molar volumes of aqueous methyldiethanolamine and its salt were determined with platinum vibrating tube densitometers over a range of temperatures from 283 K <= T <= 576 K and at pressures from 0. 1 MPa to 20 MPa. Apparent molar heat capacities were obtained using a Sodev Picker flow microcalorimeter at a pressure of 0. 1 MPa and within the temperature range of 283 K <= T <= 328 K. Apparent molar adiabatic and isothermal compressibilities at a pressure of 0.1 MPa were obtained at temperatures from 283 K <= T <= 313 K with speed of sound measurements. The experimental results were extrapolated to infinite dilution to obtain values for the standard partial molar volumes V-o, heat capacities C-p(o), and isothermal compressibilities K-T(o), The standard partial molar volumes V-o for the neutral amine and its salt show increasingly positive and negative values, respectively, at high temperatures and pressures, as predicted by corresponding states and group additivity arguments. The density model and the revised Helgeson-Kirkham-Flowers (HKF) model have been used to represent the temperature and pressure dependence of the standard partial molar properties to yield a full thermodynamic description of the system. (c) 2005 Elsevier Ltd. All rights reserved.