Liquid fuels and the fuel vapors in equilibrium with them typically differ in composition. These differences impact automotive fuel systems in several ways. Large compositional differences between liquid and vapor phases affect the composition of species taken up within the evaporative emission control canister, since the canister typically operates far from saturation and doesn't reach equilibrium with the fuel tank. Here we discuss how these differences may be used to diagnose the mode of emission from a sealed container, e.g., a fuel tank. Liquid or vapor leaks lead to particular compositions (reported here) that depend on the fuel components but are independent of the container material. Permeation leads to emissions whose composition depends on the container material. If information on the relative permeation rates of the different fuel components is available, the results given here provide a tool to decide whether leakage or permeation is the dominant mode of emission.Using well-established methods based on vapor-liquid equilibria, generalized vapor-phase correlations, and the UNIFAC model for liquid-phase nonideality, we have calculated the magnitude of the compositional difference for each species in several model fuel mixtures, as a function of temperature. In fuel C, a binary iso-octane/toluene mixture (of 1:1 volume ratio or 4:5 mass ratio), we find that the vapor is enriched in iso-octane to a 2:1 mass ratio. In ternary mixtures that contain alcohols at low concentrations (e.g., CM15), the vapor mass fraction of alcohol exceeds its liquid-phase mass fraction by a factor of three or more. In the same mixtures at high alcohol concentrations, the vapor mass fraction of iso-octane exceeds its liquid-phase mass fraction by a factor of 5 or more. The relative vapor mass fractions (on an air-free basis) of each species change with increasing temperature: the relative iso-octane fraction decreases, the relative toluene fraction increases slightly, and the relative alcohol fraction increases significantly. Results for ternary mixtures that contain MTBE and for a model indolene fuel are also presented.