The P-T-rho behavior of the CO2-CO-H-2 system was studied in the supercritical region under operative conditions close to those adopted to perform hydrogenation and hydroformylation reactions in dense CO2, thus providing new interesting information on this fluid mixture. Experiments were performed in a fixed volume reactor in the temperature range from 298 K to 343 K changing the density and the composition of the fluid phase. The one-component (Hildebrand) solubility parameter of the mixture was estimated from experimentally measured P vs T profiles, and its dependence on the density and composition of the system was analyzed to study the antisolvent effect of the permanent gases. We have found that, under adopted operative conditions, the Peng-Robinson equation of state (PR-EOS) can be used to predict with good accuracy the values of the Hildebrand solubility parameter of the binary and ternary mixtures without using adjustable binary interaction parameters. The PR-EOS was eventually used to calculate the dependence of the solubility parameter Of CO2-containing mixtures on the composition, pressure, and density. By this approach, it seems that the antisolvent effect of CO and H-2 is mainly due to the reduction of the density of the fluid phase, at fixed T and P, when the mole fraction of syn-gas components is increased.