A nonisothermal compositional model has been developed for examining nonaqueous phase liquid contamination and remediation scenarios. The governing mass balance equations and constraints have been presented, and various type of compositional formulations available have been examined. An efficient and robust formulation has been developed that addressed certain issues related to groundwater situations and overcomes numerical difficulties encountered with previous formulations. The proposed formulation collapses to that corresponding to the multiphase flow equation set when interface mass transfer is neglected. Numerical implementation of the formulation has been discussed, and example problems have been presented for benchmark and verification. The present formulation outperforms other currently used formulations for the isothermal case studied and performs at least as well as its counterparts for the nonisothermal case. Characteristics of the simulator that hinder convergence for difficult problems has been identified, and directions for even further improvements have been noted. ¿ American Geophysical Union 1995