AbstractNitrogen oxides (NOx=NO2+NO) play a role in degradation of lubricating oils in internal combustion engines. Interactions of NOxwith fuel derived species have been shown to lead to varnish and deposit formation. Mechanisms of interactions of NOxwith additive and base oil components have not, however, been investigated in detail. The present work describes investigations of the effects of NO2on oxidation processes and the fate of NOxin a model lubricant system. NO2acts as a free radical initiator which accelerates the initial rate of uninhibited hydrocarbon oxidation and increases the rate of antioxidant consumption in inhibited systems. NOxconsumption increases after the end of the inhibition period. In addition, NO2and NO react with oxidation intermediates or products in a complex series of reactions which can include direct reaction with antioxidants, formation of nitro‐oxidation products, conversion of NO2to NO and regeneration of NO2from NO. The significance of results obtained in this study with respect to mechanisms of oil degradation is discusse