The critical lines (temperatures and pressures) for the three binary mixtures ethane + sulfur hexafluoride (SF6), SF6 + propane, and carbon dioxide (CO2) + SF6, and the critical surface for the ternary system CO2 + ethane + SF6 were determined experimentally by a flow method. The binary data were compared to predictions using the predictive Soave-Redlich-Kwong (PSRK) group contribution equation of state (EoS). For the system CO2 + SF6, where no phase equilibrium information is available, the critical points were used to optimize the required PSPX group interaction parameters. Using these binary parameters, the ternary critical plane was predicted reliably.