Measurements have been made of the activity coefficients at the temperature T = 298.15 K of ammonium chloride and ammonium bromide, both in aqueous solutions of the separate salts and in five solutions containing both salts. The results have been extrapolated to give these activity coefficients in saturated solutions, and the values so obtained have been combined with published results for (ammonium chloride + ammonium bromide + water) to estimate G(m)(E), the excess molar Gibbs free energy increase on the formation of solid solutions from the two solid components. Calorimetric determinations have also been made of H-m(E), the molar enthalpy of formation, for three solid solutions. The results for G(m)(E) and H-m(E) show that at T = 298.15 K the excess molar entropy S-m(E) is positive. The values of these three excess molar functions for {xNH(4)Cl + (1 - x)NH4Br}(s.s.) at x = 0.5 and T = 298.15 K are 934 J . mol(-1), 1116 J . mol(-1), and +0.61 J . K-1. mol(-1), respectively. Entropy values, determined calorimetrically, are available for the pure salts and for three solid solutions, and when these are combined with the results for S-m(E) at T = 298.15 K, it is found that, for all three solutions, S-m(E) at T = 0 is negative. Thus, for the solid solution of composition NH4Br0.55Cl0.45, S-m(E) at T = 298.15 K is +0.58 J . K-1. mol(-1) while at T = 0 it is -1.34 J . K-1. mol(-1). It is difficult to assess the probable error associated with these values of S-m(E)(T-->0), and it may amount to +/-0.8 J . K-1. mol(-1). Nevertheless, the results suggest that there is some local positional order of the anions in the solid solutions, but that, at T = 0, there is little if any residual orientational disorder of the ammonium ions.