The influence of CO2 on the carbonylation of methanol into methyl formate was investigated with two classes of catalytic systems : MeO- and epoxide-amine combinations. In the absence of CO2, NaOMe is the more active, but its efficiency is rapidly destroyed by CO2. On the other hand, the epoxide-amine system is much less sensitive to the presence of CO2. Different classes of amines and epoxides have been tested. Tertiary alkyl amines and terminal epoxides are the most efficient. Phosphines associated with an epoxide have also been found to be able to catalyse this reaction. On the basis of P-31 NMR studies, a plausible mechanism which can also be applied to amines is proposed for the reaction.