Critical temperatures and pressures of nominal reacting mixture in synthesis of dimethyl carbonate (DMC) from methanol and carbon dioxide (quaternary mixture of carbon dioxide + methanol + water + DMC) were measured using a high-pressure view cell. The results suggested that the critical properties of the reacting mixture depended on the reaction extent as well as its initial composition (initial ratio of carbon dioxide to methanol). Such information is essential for determining the reaction conditions when one intends to carry out the synthesis of DMC with CO2 and methanol under supercritical conditions. Critical temperatures and pressures of nominal reacting mixture in synthesis of dimethyl carbonate (DMC) from methanol and carbon dioxide (quaternary mixture of carbon dioxide + methanol + water + DMC) were measured using a high-pressure view cell. properties of the mixture mixture depended on the reaction extent as well as its initial composition (initial ratio of carbon dioxide to methanol). Such information is essential for determining the reaction conditions when one intends to carry out the synthesis of DMC with CO2 and methanol under supercritical conditions.