Department of Chemistry, University College of Science, Calcutta 700009, India Manuscript received 30 April 1974; revised 24 April 1976; accepted 29 April 1975 The ionic interaction coefficients of a number of electrolytes of different valence types have been calculated by the use of the Bronsted-Scatchard-Guggenheim equation, utilising the activity coefficient values of the solutions of pure electrolytes. Herefrom have been calculated the net ionic interaction coefficients for mixtures of two electrolytes in solution, e.g. those occurring in the expressions for the thermodynamic equilibrium constant in ion-exchange studies. The total mixture molarity in these (intended constant-ionic-strength) exchange studies having had per­force to be varied within appropriate (albeit smallest possible) ranges for various reasons, especi­ally in the case of heterovalent ion exchange, the variation of the net interaction coefficients over the mixture molarity ranges involved, is of interest to consider. This has been shown graphically, in the case of sixteen cation and anion exchange systems (μ ~ 0.1) investigated.