-The reactivity of carbon dioxide with methanol to form dimethyl carbonate was studied in the presence of the n-butylmethoxytin compounds n-Bu 3 SnOCH 3 , n-Bu 2 Sn(OCH 3 ) 2 , and [n-Bu 2 (CH 3 O)Sn] 2 O. The reaction occurred under solventless conditions at 423 K and was produced by an increase in CO 2 pressure. This beneficial effect is primarily attributed to phase behavior. The mass transfer under liquid-vapor biphasic conditions was not limiting when the system reached the supercritical state for a CO 2 pressure higher than 16 MPa. Under these conditions, CO 2 acted as a reactant and a solvent.