The determination of critical points of mixtures is very important for using supercritical fluid as a clean solvent in reaction process. The critical temperatures, T-c, and pressures, P-c, are reported here for mixtures for the hydrogenation of propene in CO2 at various stages of reaction and for different reactant concentrations. These data were measured by direct observation of the critical phase transition. Our results show that the critical pressure for the mixture with an initial mole fraction of 0.40 decreases by ca. 80 bar along the reaction path from 100% reactants to 100% products. The critical temperature increases by only 7 K during this process. The Pena-Robinson equation of state has been used to calculate both critical points and P-T phase diagrams for the multicomponent mixtures using only the interaction parameters regressed from the binary subsystems. The calculated results are used to understand the phase behavior problem for reactions in supercritical fluids.