The interaction of ammonia molecules with the (100) surface of MgO has been investigated on uniform powders and single crystal surfaces using volumetric isotherm and low energy electron diffraction measurements in the temperature ranges of 170-205 and 90-120 K, respectively. The kinetics of condensation and the isosteric heat of adsorption have been measured. The results show that there is a strong repulsive interaction between NH3 molecules within the adsorbed monolayer. No ordered structure has been found by low energy electron diffraction up to monolayer coverage. The comparison of our experimental value of the isosteric heat of adsorption at low coverage and at monolayer completion with the calculated value using semiempirical potentials found in the literature leads to an estimated value of the effective surface ionic charge q/e = 1.25.