Improved oxidation methods are provided wherein a reaction mixture comprising a substrate to be oxidized (e.g., phenols, alkenes) and an oxidation catalyst (typically dispersed in an organic solvent system) is supplemented with a compressed gas which expands the reaction mixture, thus accelerating the oxidation reaction. In preferred practice pressurized subcritical or supercritical carbon dioxide is used as the expanding gas, which is introduced into the reaction mixture together with an oxidizing agent. The inventive methods improve the substrate conversion and product selectivity by increasing the solubility of the oxidizing agent in the reaction mixture.