A laboratory test procedure was developed, and used to evaluate the deposit-forming tendencies of liquid fuels on a metal surface, and to identify deposit precursors in fuel. The impetus for this work was deposit formation in multiport fuel injection(MPFI) systems. Results from our laboratory test correlated well with those from engine dynamometer tests. Deposit formation is shown to be caused by the oxidation, condensation, and precipitation of unstable hydrocarbon species in the fuel. The immediate precursors for deposit formation were determined, based on liquid chromatographic separation and GC/MS analysis, to be oxygenated hydrocarbons included in the fuel.