The formation of NO2 in the cylinder of a diesel engine has been investigated using a total cylinder sampling technique and a simple kinetic model. Exhaust measurements of NO2 as a function of equivalence ratio and as function of time after engine start were made. Samples obtained by total cylinder sampling from an operating direct injection diesel engine showed NO2/NO ratios of 25 to 50%. This is much higher than the 1 to 3% which was measured in the exhaust. Simulations of the sampling process indicate that conversion of NO to NO2 is at least partially responsible for the high NO2/NO measurements. However, the processes which produce the NO to NO2 conversion during the sampling also occur during normal combustion. This may lead to high NO2 concentrations during the combustion cycle which are then lowered during the expansion to the measured exhaust concentrations. Simple kinetic models of combustion processes similar to diesel combustion indicate that NO2/NO ratios greater than 1 may be possible during the cycle. Measurements of the NO2/NO ratios in the exhaust increase markedly as the equivalence ratio is decreased, increasing from 0.01 to 0.62 as the equivalence ratio is reduced from 0.6 to 0.07. The concentration of NO2 in the exhaust peaked at an intermediate equivalence ratio of 0.24. Measurements of the NO2/NO ratio as a function of time indicate that 45 to 60 minutes is required for stabilization.