A compositional simulator is developed for application to the analysis of contamination and remediation of groundwater systems. Simultaneous flow of three fluid phases (water, gas, and organic) is modeled. Interphase partitioning and transport of an arbitrary number of organic and inorganic components can be simulated. Phase densities are functions of pressure and phase composition. The model includes several numerical options, ranging from fully implicit with first-order upstream weighting to implicit in pressure, explicit in saturations and concentration with third-order upstream weighting. The model is verified to the extent possible with analytical solutions for simplified cases of multiphase flow and contaminant transport. The accuracy and efficiency of the various numerical options used in the model are illustrated. ¿ American Geophysical Union 1993