The surface modifications of MnO2/CeO2 catalyst during catalytic wet oxidation reaction were shown to be mainly due to carbonaceous deposits. Detailed characterization of the deposits was carried out using temperature-programmed oxidation coupled with mass spectrometry detection (TPO-MS), X-ray photoelectron spectroscopy, (XPS) and static secondary ion mass spectrometry (SSIMS). The TPO-MS provided evidence that the nature of the carbonaceous deposits was dependent on the reaction temperature and time. The XPS investigation revealed the complex chemical composition of the organic material on the catalyst surface. The Cls region exhibited aromatic/graphitic, aliphatic, and oxygen-bearing carbon. The relative contribution of the aromatic/graphitic carbon increased with reaction time. The Ols region was composed of a strong signal attributable to the inorganic oxides and signals with moderate intensity stemming from adsorbed water, surface hydroxyl, and organic oxygen from alcohols or esters in agreement with the Cls peak reconstruction. The static SIMS analysis confirmed the existence of long aliphatic chains on the catalyst surface, together with aromatics of low polycondensation level indicating a low level of graphitization of the deposits.