A simplification of Wertheim’s theory is described by which the extents of association of all species may be efficiently determined by solution of a single nonlinear equation rather than a nonlinear system of equations. This simplification should facilitate the implementation of Wertheim’s theory into chemical engineering process simulators for equations of state like the statistical associating fluid theory and the simple equation of state for associating mixtures of Elliott et al. (Ind. Eng. Chem. Res. 1990, 29, 1476). The computation speed for an associative equation of state is about 2-3 times slower than that for the Soave equation when this simplification is applied, regardless of the number of components.