Dissociation constants of NH4+ ion on the molality basis (K-a,K-m = [H+][NH3]/[NH4+]) were determined at 298.15 K in aqueous NH4NO3 solutions at different ionic strengths (I = 0.5-10.8 mol.kg(-1)) by means of a glass electrode. The pK(a,m) values increase gradually with increasing ionic strength. The activity coefficients of NH3, gamma(m,NH3), on the molality basis in the NH4NO3 solutions were determined by a transpiration method and were fitted to ln gamma(m,NH3) = -0.0416I(m) + 0.00194I(m)(2) (I = 0.5-10.8 mol.kg(-1)). The observed dissociation constants, pK(a,m), were compared with those calculated from the Fitter equations.