FUEL COMPOSITIONS FOR SPARK IGNITION ENGINES WHICH COMPRISE ORGANO-METAL ANTI-KNOCK ADDITIVES COMPRISE, IN ADDITION, AN ESTER OF A SELECTED TYPE TO MITIGATE THE FORMATION OF DEPOSITS IN THE COMBUSTION CHAMBER OF THE ENGINE. SPECIFIC EXAMPLES OF SUITABLE ESTERS ARE BENZYL SALICYLATE, ARYL PHTHALATES, PHENYLACETATES, BENZYL HALOGENATED ALIPHATIC CARBOXYLATES AND BENZYL NITRO- OR HALO-BENZOATES.