Proton transfer in ammonia-nitric acid clusters containing up to four component units are subject to theoretical calculation in this work. In a single ammonium nitrate unit, proton transfer between the nitric acid and ammonia unit does not occur but the two molecules are strongly hydrogen-bonded. In a cluster of two ammonium nitrate formula units [NH3HNO3](2), proton transfer does occur and the components are stabilized by ionic interactions. Ammonium nitrate solvated with single ammonia [NH3HNO3]NH3 or nitric acid [NH3HNO3]HNO3 molecules are also studied. Structural changes in the various clusters relative to the free molecules are discussed. Using population analysis, the total electrostatic interaction between the components of each cluster are calculated. It is argued that the magnitude of the total electrostatic interactions within the cluster determines whether proton transfer and ion formation takes place. Binding energies alone do not give a reliable indication of the occurrence of proton transfer.