The mixing behavior of viscosities of electrolyte solutions under isopiestic equilibrium has been studied for the first time using the Eyring's absolute rate theory and the semi-ideal hydration model. The concentration dependence of the change in molar free energy of activation for flow has been formulated and discussed based on the concept of average hydration number and the structure-making or structure-breaking nature of the ion species. Coupling of the Erying's theory and the semi-ideal hydration model yields very simple mixing behavior of viscosities of electrolyte solutions under isopiestic conditions and thus yields new simple equations for viscosity, which can provide predictions for mixed solutions using only information on the single solute solutions. The new equations have been tested by comparisons with the experimental results for 15 systems and the existing equations reported in literature. The present equations improve the predictions considerably and the agreement with experimental results is in general quite good. The effect of different contributions accompanying the mixing process on the predicted results has been studied. The viscosities of the mixed electrolyte solutions can be well predicted only by using the additive contribution from the viscosities of binary solutions under isopiestic condition. The addition of the mixing term in this simple additive equation yields better predictions for mixed solutions mainly consisting of the structure-making ions. The inclusion of the density term has little effect on the predicted results. (C) 2003 Elsevier Science Ltd. All rights reserved.