The phase behaviour of reservoir hydrocarbon fluids in gas injection processes involve continual mass transfer and composition changes. Compositional simulators can predict the phase behaviour of such complex processes, provided the equations of state are properly tuned. The tuning method must reflect the great composition changes caused by the mass transfer. Therefore, the tuning criteria must be based on matching the predicted compositions with real measured values for the processes, rather than just the saturation pressure and volumetric values. In this work, the results of extensive experiments on multiple contact gas injection processes are used for tuning the Peng-Robinson equation of state (PR-EOS). The tested fluids were well-defined hydrocarbon systems representing gas condensate and volatile oils and the injectants were nonhydrocarbons, N-2, CO2 and their mixtures, as they exist in typical flue gases. The measured quantities were the volumes and the compositions of gas and liquid phases at test pressures and temperatures. The tuning involved the critical properties and acentric factor of heavy components, the Omega values of the equation of state, as well as the binary interaction coefficients. Completely different tunings were needed for a simple volumetric match or for the more complicated composition match. The sensitivity checks performed on the tuning parameters have shown that the critical properties of heavy components greatly affect the compositions, especially those of the liquid phase.