An ab initio study (MP2/6-31+G*//HF/6-31+G*) has been performed to investigate how solvation affects the mechanism in carbonyl addition of methyllithium (MeLi) in dimeric and trimeric complexes to formaldehyde. The computations were performed on a homo-dimer of MeLi and a hetero-dimer and -trimer of MeLi and amide base. Results show that a polar solvation comprising one formaldehyde and one water molecule around the lithium ions, coordinating the reactive methyl group, causes the addition to occur according to a concerted mechanism involving a four-membered-ring transition state. However, a hydrophobic solvent and a low substrate concentration promote a nonconcerted mechanism involving ring opening of the dimers and the trimer.