This paper reports high-pressure phase equilibrium data on binary mixtures of carbon dioxide with dimethyl sulfoxide (DMSO) and ternary mixtures of DMSO, carbon dioxide and water. The data cover a range of temperatures between 280 and 370 K and pressures up to 15 MPa. Partial liquid miscibility was observed in the binary system DMSO + CO2 at high CO2 concentrations. Vapor-liquid, liquid-liquid and vapor-liquid-liquid equilibrium conditions were measured. The experimental data were modeled with a group-contribution equation of state that takes into account association effects (GCA-EOS). Self-association of water and cross-association between water and DMSO molecules were considered. A molecular group (CH3SOCH3) was used to represent DMSO, the first member of the alkyl-sulfoxide family. Pure group and binary interaction parameters for this new molecular group have been determined. Good correlation and prediction of phase equilibrium conditions were obtained. (c) 2007 Elsevier B.V. All rights reserved.