Lattice fluid models with temperature-independent segment number, interaction energy and component-specific cell volume are in use to represent pure component physical properties conveniently and accurately. However, the formulation of mixtures from pure components calls for a new lattice different from constituting pure components and a consistent method has not been presented so far. In the present study Helmholtz free energy was derived based on well-defined assumptions for physically interacting lattice fluid mixtures and was obtained in terms of pure component parameters and a new combining rule for energy parameter. When applied to azeotropic mixtures and mixtures with light components for a wide range of pressures, the free energy with the new combining rule was found to show general improvements. (c) 2007 Elsevier B.V. All rights reserved.