A modification of the extended UNIQUAC model is proposed for the description of the non-ideality of protein solutions. Here, the Staverman-Guggenheim combinatorial contribution used in extended UNIQUAC model is replaced by the Flory-Huggins term to take into account the size differences between the protein and solvent. This new model allows an excellent description of the activity coefficients in protein systems, for a large range of pH and ionic strengths, with a reduced number of parameters. (C) 2004 Elsevier B.V. All rights reserved.