The occurrence of numerical pitfalls has been pointed out for the models of equation of state, including the statistical associating fluid theory (SAFT)-based. Following our previous work on pure ILs when modeling with ePC-SAFT, this work extended the research to the IL-gas mixtures, in which the associating term was derived for predicting the critical point with PC-SAFT, and a scheme for detecting a pitfall in IL-gas mixtures was developed. Then, the scheme was applied to detect the pitfall occurrence in IL-gas (gas = CO2, H2S, CO, O-2, CH4, N-2, and H-2) systems, which are commonly encountered in developing IL-based CO 2 separation technologies. It shows that a pitfall occurs only in [C(8)mpy] [BF4]-gas (CO2, H2S, CO, O-2, CH4, N-2, and H-2) among all ILs considered in the temperature and pressure range of interest.