This work demonstrates the perfect analogy between the standard method for determining activities in non-electrolyte mixtures and the method used for the determination of ionic activities in electrolyte solutions. For the case of electrolyte solutions, the general procedure of solving three equations with three unknowns is discussed and the reasons why this procedure failed to work for the case of KCl solutions are recapitulated. A modified iteration procedure implemented in this work gives convergence of the solution after three iterations. The crucial difference between eliminating the junction potential, by combination of independent equations, and the use of a proper estimate of its value is emphasized. A new model-free calibration procedure is proposed and sample calculations of K+ and Cl- activities in KCl aqueous solutions are presented. Values of K+ and Cl- activities in KCl aqueous solutions obtained by this new procedure and by a variety of other methods and researchers are compared. (C) 2011 Elsevier Ltd. All rights reserved.