In this article, we analyze the effect of the alcohol content in gasoline on the permeability of four semi-crystalline polymers used in automobile fuel systems. More specifically, we are interested in the methanol/toluene mixture as a "binary" model of a complex gasoline to understand the selectivity brought about, in front of these solvents, by the nature of the polymer. We developed a permeation cell coupled to a chromatograph to analyze the composition of escaping toluene and methanol. These experiments allow us to demonstrate the strong "positive synergy" that exists between the flows of the methanol and the toluene when they are mixed, compared to the flows of these solvents on their own. This phenomenon is notably highlighted on polymers of very different kinds (PA12, PVDF, HDPE, EVOH) and, in light of recent theoretical developments, we can consider that this property is general, because this "Positive synergy" is a consequence of the evolution of the solubility of the mixture of solvents in the polymer film, and more particularly, of the highly positive value of the Flory-Huggins interaction coefficient between methanol and toluene. These experimental data allow one to better understand and predict the permeability behavior of the polymers in front of complex gasolines. (C) 2004 Wiley Periodicals, Inc.