A novel molecular entity vacancy model was proposed to describe thermodynamic properties in a multicomponent solution system using its binary interaction parameters only. A derivation of the model for its general expression has been shown in detail. Under some special conditions, this model may be reduced to Flory-Huggins equation, Wilson equation and non-random two liquids (NRTL) equation as well as molecular interaction volume model (MIVM), respectively, and can be verified by Gibbs-Duhem equation, and can express thermodynamic properties of partially miscible systems. The predicted activities are in good agreement with experimental data of some liquid alloys. The results show that the model is of better predictability and reliability because it has a certain physical basis. (c) 2006 Elsevier B.V. All rights reserved.