An experimental and theoretical study of HNCO oxidation has been carried out. The experiments were performed in an isothermal quartz flow reactor, and the effects of temperature, CO concentration, and NO concentration were investigated at atmospheric pressure in the temperature range 1025-1425 K. The reaction mechanism for RAPRENOx proposed by Miller and Bowman (1991) has been updated based on the present result as well as recent advances in the understanding of important elementary steps. Model predictions with the revised mechanism are in good agreement with our experimental data as well as data from the literature. Oxidation of HNCO proceeds mainly through NCO, which subsequently is oxidized to NO or reacts with NO to form N2 and N2O. This sequence of reactions is chain terminating, and for reaction to occur, radicals must be generated either by alternative oxidation pathways or by the presence of other combustibles. A chain-branching oxidation route initiated by reaction of HNCO with O2 is proposed in order to explain the observed HNCO decay in the absence of inlet CO. Addition of CO enhances HNCO oxidation and the RAPRENOx chemistry, since CO oxidation acts to replenish the radical pool. The experimental results show that the mutual presence of HNCO and NO strongly inhibits CO oxidation at lower temperatures. In addition to the chain terminating HNCO/NCO reactions, a second inhibition mechanism involving NO is necessary to explain this behavior. This mechanism is presently believed to be NO NO2 interconversion, but additional work is needed to confirm this. Further progress in the understanding of the HNCO chemistry is dependent on an accurate determination of the rate and/or mechanism of a number of key reactions, including HNCO + OH, HNCO + O2, NCO + NO and NO + O + M.