A homogeneous binary catalyst system, n-Bu 4 NBr/n-Bu 3 N, was found to be active for the synthesis of dimethyl carbonate from styrene oxide (SO), methanol, and supercritical CO 2 . Under the optimized conditions, the dimethyl carbonate yield could reach 84% at SO conversion of 98%. Several parameters were studied, i.e. catalyst precursors, reaction time and temperature, methanol/epoxide feed ratio in moles, and CO 2 pressure. The best compromise for the one-pot synthesis was achieved with an equimolar amount of n-Bu 4 NBr/n-Bu 3 N. A possible mechanism for the present n-Bu 4 NBr/n-Bu 3 N-catalyzed one-pot synthesis of dimethyl carbonate was proposed.