An alternative means of epoxidation is reported that uses environmentally benign supercritical carbon dioxide as both a solvent and reactant in combination with aqueous H2O2, which is made possible through the in situ formation of peroxycarbonic acid. Experiments were conducted at 40 degreesC and 120 bar in which cyclohexene was epoxidized to 1,2-cyclohexene oxide and 1,2-cyclohexanediol in this aqueous-organic biphasic system. Through the addition of NaHCO3 and the hydrophilic cosolvent dimethylformamide, the conversion increased from 0.4 mol % (without additives) to 12.6 mol % (with 0.1 mol % NaHCO3 and 13 mol % dimethylformamide). The results suggest that the reaction occurs within the aqueous phase, which led to investigations using the water-soluble olefin 3-cyclohexen-1-carboxylate sodium salt as a means of verifying the reaction location. Epoxidation of 3-cyclohexen-1-carboxylate sodium salt went to completion in less than 20 h at 40 degreesC and 120 bar with an epoxide yield of 89 mol % and diol Yield of 11 mol %.