Densities and vapor-liquid compositions for twelve complex mixtures (crude oil and natural gas) are calculated using a lattice fluid model equation of state. Fractions of the heavy ends are approximated as pseudocomponents defined in terms of their 'equivalent normal alkane'. The lattice fluid model equation of state of Sanchez-Lacombe is used, three parameters are required for each pure component and one binary interaction parameter (zeta(ij)) is required for each unlike pair. Binary interaction parameters are developed for 56 binary mixtures whose components are typical of those found in petroleum systems. It is seen that, even though pseudocomponent are defined in very simple manner, the predicted results for multicomponent systems are comparable to or better than many popular alternative methods.