Major and minor products of MTBE decomposition and oxidation were measured in a jet-stirred flow reactor operated at high pressure near to the conditions typical of end-gases in spark-ignited engines. Results showed that the conversion of MTBE was enhanced by the oxidative environment which activates the formation of MTBE radicals, the decomposition of which leads to the formation of an equimolar mixture of i-C4H8 and CH3OH. At higher inlet temperatures, also the genuine MTBE oxidation through the classical ＇＇peroxy＇＇ equilibrium typical of the low-temperature oxidation of hydrocarbons was observed from both phenomenological and chemical point of views. Noticeable heat release and a cool flame phenomenology were observed up to the complete conversion of MTBE and characteristic products of O-2 addition to MTBE redicals were detected (heterocyclics and light oxygenated products). However, the largest part of MTBE is converted by thermal degradation in isobutene which, being a species relatively inert to oxidation, is the cause of the anti-knock effect of MTBE.