In this work we present the results of a study of the solvent effects in the protonation of ammonia from a hydronium ion. The potential energy surface in the gas phase and the free energy surface in a continuum have been characterized at the HF/6-311G** level. Stationary points have been also relocated at the MP2/6-311G** level. The results obtained with the continuum model are compared with those of a discrete-continuum model in which five additional water molecules are placed around the proton acceptor and proton donor molecules. Both from the electronic and geometric points of view, solvent effects enforce the N-H bond while weakening the O-H bond. At fixed N-O distances, solvent effects thermodynamically favor the proton transfer from hydronium to ammonia but make the process kinetically slower by increasing the energy barrier.