Ag+ beta-alumina has been employed as a solid electrolyte in order to measure the activities of silver for Ag2S in equilibrium with controlled sulphur atmospheres. Electromotive-force measurements were made in the temperature range 811 K to 1112 K using the galvanic cell : PtAgAg+ beta-Al2O3Ag2S, MoS2(1 - x(2) - x(3))N-2 + x(2)H(2) + x(3)H(2)SPt. Manipulation of the results yielded for the standard molar Gibbs free energy of formation of Ag2S with respect to pure Ag(s) and S-2(g) at p(o) = 101.325 kPa : 2Ag(s) + 1/2S(2)(g) = Ag2S(s), Delta(f)G(m)(o)/(J.mol(-1)) = {-85326 + 35.15(T/K)} +/- 2500.