Experimental investigations and kinetic interpretation indicate the presence of several peroxidic species during the oxidation of n-dodecane. The oxidation was performed either in the gaseous phase in a flow system, or with a liquid/gas interaction in a bulb, because in many combustion processes liquid and gas coexist during preignition. The different peroxides were identified by mass spectrometry. Simple hydroperoxides with two oxygen atoms per molecule were observed at relatively low temperatures above 370 K with liquid and gas present. Ketohydroperoxides with three oxygen atoms per molecules are formed at similar to 500 K. Di-, and even tri, ketohydroperoxides with four or five oxygen atoms per molecule are also present under these experimental conditions at 518 K. Their formation is explained by isomerization reactions of alkoxy radicals OR-2HO bearing a carbonyl group; these stem from the decomposition of carbonyl-hydroperoxides. Depending on the experimental conditions, the nature of the hydroperoxides is different and the respective reactions of these species should be introduced in low temperature oxidation mechanisms. These reaction schemes should include the isomerization of peroxy radicals RO2 and of ketoalkoxy radicals OR-2HO, leading to the formation of peroxides with several carbonyl groups.