Typically, the oxidation of organic substrates with H2O2 and water-soluble catalysts involves biphasic systems in which the substrate resides in an organic phase and the reaction occurs in the aqueous phase. We demonstrate how the miscible region (in P-T-x space) for water/solvent/ CO2 ternary systems can be exploited to overcome interphase mass-transfer limitations of such biphasic systems and perform homogeneous oxidations of organic substrates using water-soluble catalysts and oxidants. By employing CO2-expanded CH3CN/H2O2/H2O mixtures, a variety of olefins (cyclohexene, styrene, 1-methylcyclohexene, 4-methylcyclohexene) were oxidized homogeneously with high (>85%) epoxidation selectivities. An order-of-magnitude enhancement in epoxidation rates was achieved with the addition of pyridine to the homogeneous system, at pressures that are an order of magnitude lower than those needed in the biphasic system described in the literature. Plausible reaction pathways that involve the formation of peroxy carbonic acid as a catalyst are discussed.