A new speciation-based group contribution model for activity coefficients is proposed to estimate the equilibrium properties of aqueous solutions containing electrolytes. The chemical part of the model accounts for the hydration equilibrium of water and ions with the formation of ion n-water complexes in a single stage process; the hydration number n and the hydration equilibrium Constant K are the two independent parameters in this part. The physical part of the model is the UNIFAC group contribution model for short-range interactions. Each ion is considered as a group. Long-range interactions are accounted for by a Pitzer contribution (Debye-Huckel theory). The model is compared with experimental data at 25 degrees C including water activity. osmotic coefficients, activity coefficients, and pH of binary diluted and concentrated electrolyte solutions (up to 20 mol kg(-1) for NaOH, 16 mol kg(-1) for HCl, etc.). (C) 2009 Elsevier B.V. All rights reserved.