Deposition of silver from electrolyte solutions is of major importance in the industrial applications of photographic development and electroplating. Prior to the kinetic study of the reduction of silver thiosulphate complexes, the concentrations, activities and activity coefficients of all components formed in AgNO3-Na2S2O3-NaNO3 solutions are calculated, starting from the measurement of the equilibrium potential. In view of the high ionic strength of the solutions (greater than 0.1 mol kg-1), the ion interaction model is applied for the estimation of the activity coefficients, inevitably imposing the use of an iterative calculation routine. The activity coefficients are shown to comply with known thermodynamic laws, supporting the appropriateness of the model, together with the approximations.