We present a thermodynamic model of mixtures in which the Helmholtz energy is computed from an extended corresponding states (ECS) method with a correction term. The ECS model is based upon the van der Waals one-fluid theory with pure-component shape factors represented as functions of reduced temperature and density. The correction term is given by a local-composition model with a coordination number taken from lattice gas theory. Densities and speeds of sound of binary mixtures were calculated with deviations within +/-0.20%, while deviations in bubble-point pressures were within 2% on average. (C) 2004 Elsevier B.V. All rights reserved.