A kinetic study of the methanolysis of t-butylbromide and benzoylchloride in methanol-acetonitrile mixtures is reported. First order rate constants for the methanolysis of t-butyl bromide show maximum at $X_{MeOH}$=0.75∼0.9 and 25∼$50^{circ}C$. Apparent second order rate constants for the methanolysis of benzoylchloride also show maximum at $X_{MeOH}$= 0.6∼0.95 and 12∼$26^{circ}C$. The maximum rate is ascribed to the solvent structure change; the addition of acetonitrile to methanol perturbs the methanol structure increasing the free methanol molecules available to stabilize the transition state for the methanolysis of t-butylbromide and benzoylchloride. It has been shown that methanol acts as nucleophilic and electrophilic catalyst upon methanolysis of t-butylbromide and as electrophilic catalyst upon methanolysis of benzoylchloride.