An understanding of homogeneous catalysis in supercritical fluids requires a knowledge of the phase behavior and the variation in critical point as the reaction proceeds. In this paper, the critical temperatures, T-c and pressures, P-c, have been measured for a considerable number of mixtures representing the various stages of the hydroformylation reaction of propene in supercritical CO2 and different reactant concentrations. Critical point data have also been measured for all of the binary mixtures of the components (CO2, H-2, CO, propene, n- and isobutyraldehyde) which are not available from the literature or can be deduced from published data. We use the stoichiometry of the reacting system to simplify greatly the phase behavior problem by defining a path through the otherwise multidimensional "phase space". Satisfactory modeling of the data (0.3% in T, and 3.0% in P-c) has been achieved using the Peng-Robinson equation of state and ignoring all binary interactions which do not involve CO2. The model is used to explore the strategies needed to avoid phase separation in continuous and batch reactions. At a given temperature, a batch reactor may need to be run under much higher pressures than a flow reactor if single-phase conditions are to be preserved throughout the course of the reaction. Most of the critical point data were measured acoustically, but a selection of points were validated using more traditional view-cell procedures.