The recently introduced UNILFAC-vdW-FV model has been extended to account for the molecular weight effect in water-PEO systems, one of the most important polymer systems. New groups have been defined and new interaction parameters have been regressed. The UNIFAC-vdW-FV model is able to predict the significant end-group effect in these low molecular weight water-PEO systems. (c) 2005 Elsevier B.V. All rights reserved.