We report a theoretical study on ester aminolysis reaction mechanisms in aqueous solution. It is believed, in general, that the formation of a zwitterionic intermediate plays a key role in the aminolysis process whose rate determining step is the formation or breakdown of such an intermediate, depending on pH. However, the reaction mechanism has been object of some recent experimental controversy, We have investigated the model reaction ammonia + formic acid. Stepwise and concerted processes have been studied, Static and dynamic solvent effects have been analyzed by using a dielectric continuum model in the first case and molecular dynamics simulations together with the QM/MM method in the second case. The results show that a zwitterionic structure is always formed in the reaction path although its lifetime appears to be quite dependent on solvent dynamics.