Homogeneously catalyzed propylene hydroformylation in supercritical carbon dioxide (SCCO2) using Co-2(CO)(8) pre-catalyst was studied in batch reactors. The experiments were carried out at temperatures of 66 to 108 degrees C, and pressures of 90.6 to 194.1 bar. The phase behavior of the reaction mixture was monitored in a cell with a sight glass to ensure that the reactions were carried out in the single phase region. The product selectivity was determined at different reaction conditions. At a fixed temperature, the selectivity increased with pressure and at a fixed pressure it decreased with temperature. By chemical equilibrium calculations it was shown that the changes in selectivity were not chemical equilibrium controlled. The partial molar volumes of two isomers of the product were determined experimentally and after fitting the interaction parameters for the Peng-Robinson equation of state from partial molar volume data, the partial molar enthalpies were predicted from the same equation. It is shown that the selectivity increase with pressure at constant temperature can be explained in terms of the difference of the partial molar volumes of the isomers and the selectivity decrease with temperature at constant pressure can be explained in terms of the difference of the partial molar enthalpies of the two isomers.