In this work, the Ion Pair Modified Ghotbi-Vera Mean Spherical Approximation (IP-MGV-MSA) model was proposed to correlate the mean ionic activity coefficients (MIAC) for a number of symmetric and asymmetric aqueous electrolyte solutions at 25 C. The new model is based on the recently proposed MGV-MSA model by Mortazavi-Manesh et al. In the IP-MGV-MSA model, the effects arising from the ion pair formation in the electrolyte solution was taken into account. Also, in the proposed model, while the cation diameter as well as the relative permittivity of water was considered to be dependent on electrolyte concentration, the anion diameter was independent of electrolyte concentration. The results obtained from the IP-MGV-MSA model showed that the model can accurately correlate the MIAC of single electrolyte solutions, in which ion pairs are formed. These results were also compared with those obtained from the GV-MSA and MGV-MSA models. The comparison showed that in the electrolyte solutions in which ion pairs are formed, the IP-MGV-MSA model can give more superior results than those obtained from the MGV-MSA and GV-MSA models. In the proposed model, the equilibrium constant of ion pair formation, K, is an adjustable parameter and can be obtained using the experimental data of the MIAC for electrolyte solutions. It should be noted that the values for the MIAC obtained from the IP-MGV-MSA model were converted from the McMillan-Mayer (MM) framework to that of the Lewis-Randall (LR) in order to be compatible with the experimental data. (C) 2007 Elsevier B.V. All rights reserved.