Ab initio quantum chemical calculations of the reaction H3O+ + NH3 --> H2O + NH4+ have been performed using the following procedures : MP2/6-31G(d,p), HF/6-31G(d,p) and HF/3-21G. Two-dimensional representations of the potential energy functions were constructed at MP2/6-31G(d,p), and this is the most detailed and precise potential surface of this system to date. Particular features of the three surfaces which may be important in forthcoming ah initio direct dynamics calculations are discussed. Some significant structural and energetical differences are noticed. When compared with MP2/6-31G(d,p) the results shows that HF/6-31G(d,p) and HF/3-21G give slightly less accurate representations of the reaction.