The critical transitions of 21 binary mixtures containing ammonia + either an n-alkane (methane to octadecane) or a gas (hydrogen, helium, nitrogen, argon, carbon monoxide and sulphur hexafluoride) are calculated using the Heilig-Franck equation of state with the aid of two adjustable combining rule parameters xi, zeta). The calculations are in very good agreement with experimental data in the pressure range 3 - 90 MPa. For Type II mixtures, a progressive decline in the xi parameter is observed as the size difference between the component molecules is increased whereas the zeta value passes through a maximum. A disparity in molecular size between component molecules does not seriously deteriorate the quality of agreement between theory and experiment. The theoretical analysis of Type III systems is also quantitatively accurate. At high pressures, "gas-gas" immiscibility of the second kind is predicted for the ammonia + sulphur hexafluoride mixture.