Dissociation constants of NH4+ ion, K-a,K-c = cH((T))(+)cNH(3)/cNH(4)(+), where c denotes molarity and cH((T))(+) = cH(+) + cHSO(4)(-), were determined at 298.15 K in aqueous (NH4)(2)SO4 solutions at different ionic strengths (1 = 0.75-7.5 mol dm(-3)) by potentiometry using a glass electrode. The K-a,K-c values were converted to those on the molality basis by use of the densities of the corresponding solutions. The activity coefficients of ammonia, gamma(c.NH3) and gamma(m.NH3), respectively, on the molar and molal bases, were determined by a transpiration method. The observed dissociation constants on the molality basis, pK(a,m), were compared with those calculated from the Pitzer equations.