The gradient theory of inhomogeneous fluid is for the first time used to study the fluid-fluid interfaces of real petroleum mixtures (gas condensate and oils) at reservoir conditions. The theory is combined with the volume-corrected Peng-Robinson equation of state which allows a good description of the vapour-liquid equilibria of these mixtures. First, it is demonstrated that a correlation that was derived previously to compute the influence parameters of pure hydrocarbons can be also used to obtain the influence parameters of cuts and pseudocompounds present in the petroleum fluids. It turns out that the gradient theory allows estimates of the surface tension in excellent agreement with the experimental data. In any case, it is found to be much more superior to the traditional parachor method that systematically underestimates the surface tension of these mixtures. Furthermore, unlike the parachor method, the gradient theory proves to be nearly insensitive to the compositional description of the fluid provided that the number of pseudocompounds is not too small (N > 5). (C) 2007 Elsevier Ltd. All rights reserved.