Engine and laboratory experiments are performed to understand the formation of deposits during ignition delay. An experimental Diesel engine allows to observe deposit formation as a global phenomenon including homogeneous and heterogeneous reactions as well as interaction between liquid and gaseous phases. A CFR engine is used to work under the same global conditions but without the liquid phase. A flow reactor enables to investigate the effects of defined and controlled temperatures and fuel/air ratios. Results show that deposit formation is facilitated by temperatures of the oxidized gases within the 290 - 350 degrees C range and by the presence of the liquid phase. In this temperature range, carbonyl-hydroperoxides ORO2H, RO and RO2 radicals are known to be present and to play an important role. Various species are identified (carboxylic acids, aldehydes, ketones, dione, furanone) by GC/MS. Homogeneous and heterogeneous reactions of RO and RO2 radicals yield analogous types of compounds. All these results allow to assume that deposit formation is linked to the low temperature reactional range where the isomerization of RO and RO2 radicals and carbonyl-hydroperoxides ORO2H is determinant.