Abstract: A formula for the calculation of local composition is derived from Guggenheims quasi-chemical equation. A thermodynamic model based on this formula,which is called the Quasi-Chemical Local Composition model (QCLC), is proposed. The group contribution treatment is introduced into this model and the group interaction parameters for alkane, aromatics, ketone, alcohol and water are obtained by fitting the vapour-liquid equilibrium data of binary systems. A comparison of the predictions with experimental results for ternary systems indicates that the model is better than UNIFAC.