The program computes phase envelopes for two binary mixtures containing ethane and benzene (20 % ethane and 80 % benzene; 70 % ethane and 30 % benzene). This binary system exhibits a peculiar behavior called retrograde condensation (e.g. We get condensation as we increase the temperature or decrease the pressure). We use the SRK equation of state to compute equilibrium constants. The critical points were obtained from HYSYS 3.2. This problem was presented in Thermodynamique, Application au Génie Chimique et à l'Industrie Pétrolière by Jean Vidal, Technip, 1997.