It is shown, using the lattice theory and regular solution assumption, that the activity coefficient of each component at a liquid-vapour interface is equal to the second root of that component's activity coefficient in the bulk liquid phase. Making use of this important finding, an equation for prediction of liquid-vapour surface tension in multi-component systems has been developed. The new equation needs the temperature, composition of the liquid phase, activity coefficient and partial molar surface area of each compound as input data. The equation does not include any empirical parameter and has no limitation with the number of components. Comparing the predicted results with the experimental data of various systems revealed the reliability of the new model. (c) 2005 Elsevier B.V. All rights reserved.