This study was conducted to better understand the processes of carbonaceous deposits formation in the piston grooves of direct injection Diesel engines. An experimental investigation of parameters affecting the engine oil degradation in the first groove of a Diesel engine was carried out to shed light on the formation of these carbonaceous deposits. A dedicated reactor was designed to reproduce the parameters that exist in the first groove of a Diesel engine used under exhaust gas recirculation conditions, and squalane was used to model the lubricant. The relationship between oil degradation and environmental conditions ( temperature, pressure, time, oxygen content and components of the gas in the groove) was clarified by characterizing the degradation levels of the squalane molecule. A general mechanism was proposed to explain the formation of carbon deposits in the first piston grooves of Diesel engines depending on the surrounding atmosphere. The influence of pressure on degradation conditions usually investigated under atmospheric conditions was proposed for the first time to understand processes responsible for carbonaceous deposit formation. (c) 2012 Elsevier Ltd. All rights reserved.