We have used Gibbs ensemble and histogram-reweighting Monte Carlo techniques to obtain the phase behavior of water and aqueous mixtures at high pressures. These calculations are relevant for the design of high-pressure separation processes, such as supercritical fluid extraction and supercritical water oxidation. In particular, we have obtained the phase behavior of several simple point charge models of water, as well as polarizable versions of these models. None of the models studied reproduce the experimentally observed phase behavior and critical parameters of water. The binary mixture water/carbon dioxide at high pressures has also been studied. Good agreement for this mixture has been obtained without use of any adjustable parameters for the mixture interactions, thus suggesting that the predictive power of molecular-based methods is greater than that of purely phenomenological approaches, even when using imperfect pure component intermolecular potential models. (C) 1998 Elsevier Science B.V. All rights reserved.