To quantify the role played by the proton transfer reaction (NH4++Cl(-)double left right arrowNH(3)+HCl) on the phase behavior of ammonium chloride (NH4Cl) we have developed a simple interaction potential (independent atom model) to implement in a molecular dynamics simulation (MD). In using one phase and two phase MD simulations, the solid-solid and solid-fluid coexistence lines have been located. When the proton transfer reaction is found to play no role in the stability and coexistence of the solid phases (ionic), the occurrence of covalent species (NH3, HCl) in the fluid phase has an important influence on the melting process and on the solid-fluid equilibria. In particular, a solid-fluid coexistence is found between a purely ionic solid and a fluid mixture composed of ionic and covalent species in nearly equal proportions.