The gradient theory of fluid interfaces is applied to compute the surface tension of various binary and ternary mixtures made up of a gas (carbon dioxide, nitrogen or methane) and hydrocarbons. The inputs of the theory are the Helmholtz energy density of the bulk homogeneous fluid and the influence parameters of the interfacial inhomogeneous fluid. The volume corrected Peng-Robinson equation of state (PR-EOS) is used to determine both the Helmholtz energy density and the bulk properties. The adjustable parameters of the equation of state are used to improve the description of vapour-liquid equilibria (VLE). The influence parameters are obtained from a correlation presented in a previous paper. It is shown that the geometric mixing rule for the influence parameters is the most efficient. This rule is used without any adjustable coefficients, which makes the method predictive. For carbon dioxide/hydrocarbon and methane/hydrocarbon mixtures, the predictions are in very good agreement with the available experimental data. The poorest estimates are obtained for the nitrogen/hydrocarbon mixtures (AAD similar to 10%). However, even in that case, the gradient theory remains highly superior to existing traditional methods that can be used for such systems. (C) 2004 Elsevier B.V. All rights reserved.