The work summarized here demonstrates a new concept for exploiting dense phase CO2, media considered to be ''green'' solvents, for homogeneous catalytic oxidation reactions. According to this concept, the conventional organic solvent medium used in catalytic chemical reactions is replaced substantially (up to 80 vol %) by CO2. at moderate pressures (tens of bars), to create a continuum of CO2-expanded solvent media. A particular benefit is found for oxidation catalysis; the presence Of CO2 in the mixed medium increases the 02 solubility by ca. 100 times compared to that in the neat organic solvent while the retained organic solvent serves an essential role by solubilizing the transition metal catalyst. We show that CO2-expanded solvents provide optimal properties for maximizing oxidation rates that are typically 1-2 orders of magnitude greater than those obtained with either the neat organic solvent or supercritical CO2 as the reaction medium. These advantages are demonstrated with examples of homogeneous oxidations of a substituted phenol and of cyclohexene by molecular 02 using transition metal catalysts, cobalt Schiff-base and iron porphyrin complexes, respectively, in CO2-expanded CH3CN.