Recent measurements of NOx emissions from a 2.2L HSDI Diesel engine have suggested that NO decomposition may be important at high load . In interpretation of these data, Mellor et al.  determined that the nitrous oxide and extended Zeldovich mechanisms are both important pathways for NO formation and decomposition. To further examine the importance of NO decomposition in Diesels, results from tests that involve the injection of pure NO into the intake air of a 2.4L HSDI Diesel are presented.The effects of engine speed and load on the relative importance of NO decomposition are directly discernable from graphs of engine–out NOx versus engine–in NO for speed and load sweeps. The importance of NO decomposition is found to increase with engine load, while engine speed exhibits a tradeoff. Furthermore, the results indicate that the reverse of the Zeldovich mechanism dominates the NO decomposition process. Findings from similar tests on an IDI Diesel and a SI engine also reveal that the reverse of the Zeldovich mechanism dominates the NO decomposition process and the importance of NO decomposition increases with equivalence ratio, as in the present tests.