Measurements of the critical properties p(c), rho(c), and T-c for the pure substances CO2, CHF3, SF6, and for (CO2 + CHF3) and (CHF3 + SF6) were performed by observation of the critical opalescence in a static equilibrium cell. The experimental data were compared to values given in the literature. The experimental results for the binary mixtures are described by the Peng-Robinson equation of state, using only the experimental data for the critical pressure and the critical temperature of the pure components and of one mixture of nearly equal amounts of substance. For the binary systems investigated, this equation of state describes the critical loci, the vapour-liquid equilibrium, and the dependence of the supercritical density on temperature and pressure rather well.