The development of a methodology in order to study the medium effects on solution equilibria is presented in this series. This methodology uses Bromley’s approach for the estimation of activity coefficients of species in solution. In this first part, the values of the interaction parameters obtained by means of the so-called Modified Bromley’s Methodology (MBM) are given for more than 80 strong electrolytes for both the molar and the molal scales at 25-degrees-C. The parameters are calculated from the recommended values of the mean activity coefficients in the molal scale collected in the literature.