A single cylinder engine was used to collect engine combustion chamber deposits insitu, and to investigate the influence of fuel composition on combustion chamber deposit formation. High-boiling aromatic compounds were found to contribute greatly to deposit formation, while olefinic compounds did not show any significant deposit-forming tendencies. In a low surface temperature regime, deposit formation increased with boiling point of aromatic dopants added to the base fuel. Various analytical techniques (FTIR, GC/MS, EPMA, SEM) were utilized to characterize carbonaceous deposits in an early stage of formation. Oxidized hydrocarbon species (ketones, carboxylic acids, lactones, esters), metal carboxylates, and decomposition products of oil additives were found to be major building blocks of deposits. The oxidized hydrocarbon species formed in a preflame region are condensed/adsorbed on a relatively cold surface to form combustion chamber deposits.