A new method is developed to represent the phase behavior of multicomponent systems. This method uses fewer pseudocomponents than true components, but unlike conventional methods in which pseudocomponents are often chosen arbitrarily, the method uses regression analysis to find a ″best″ set of pseudocomponents. The method is applied to two surfactant systems of the type used for tertiary oil recovery. One system contains a crude oil (from the North Burbank Unit, Osage County, OK) and the other contains a pure hydrocarbon, 1-phenyltetradecane.