Densities and equilibrium constants have been measured for aqueous solutions of amines, alkanolamines and hydroxy-carboxylic acids at temperatures up to 350°C and pressures up to 20 MPa. Densities were measured using a high-temperature and high-pressure vibrating tube densimeter. Apparent molar volumes, V [straight phi] were calculated for each solute from the experimental densities and extrapolated to give standard partial molar volumes, V02 . The experimental values have been corrected for hydrolysis or dissociation, where necessary, and the variation of the volumes with temperature and structure of solutes were discussed. With the exception of glycolic acid, all the other organic nonelectrolytes studied showed normal behavior, namely V02 becomes increasingly positive as the critical point of water is approached. The equilibrium constants were measured using UV-visible spectroscopy and thermally-stable colorimetric pH indicators up to the temperature, t = 350°C and pressure up to 20 MPa. A specialised flow through cell was built for use in conjunction with UV-visible spectroscopic techniques for equilibrium constant measurements. The flow cell consists of a platinum liner and titanium casing and was tested for use in the temperature range up to 350°C and pressure up to 20 MPa. A new functional group additivity scheme for standard partial molar volumes of aqueous organic solutes has been developed using the high-temperature experimental data from this work and the limited available data from the literature. The group additivity contribution of the -CH3, >CH2, -CH, -NH2, >NH, >N-, -OH and -COOH functional groups were obtained as a function of temperature and pressure. The additivity method utilizes the features of the equation of state by O'Connell 'et al'. (1996) and is similar to that of Yezdimer 'et al'. (2000), used over a wide range of temperature and pressure with emphasis on the near-critical region. The new group additivity model developed in this work is more accurate than the existing group additivity models for predicting standard partial molar volumes up to at least 325°C.