A thermodynamic model using the predictive Soave-Redlich-Kwong (PSRK) group contribution method to calculate the fugacities of all components in the vapor and liquid phases in equilibrium with the coexisting hydrate phase is proposed. Since the PSRK method together with the UNIFAC model takes the gas-gas interaction in the vapor and liquid phases into account, the phase equilibria of mixed gas hydrates can be successfully reproduced. This approach greatly improves upon the accuracy of the modified Huron-Vidal second-order (MHV2) model, especially for three-guest hydrate systems. Based on experimentally determined X-ray data, an accurate representation for the molar volume of the structure I (sI) hydrate is provided and used for predicting the equilibrium dissociation of methane hydrate at high pressures. Using this correlation, it is possible to reduce noticeable errors in dissociation predictions of high-pressure hydrate formers. Complete phase behavior, including a new quadruple point, which is predicted to be 272.6 K and 7.55 MPa, for cyclopropane hydrate, is presented by the proposed model calculation. (C) 2004 American Institute of Chemical Engineers