A number of reduced chemical kinetics schemes are compared for prediction of NOx emissions from a slow-speed marine diesel engine, using a zero-dimensional model. The kinetic evolution of NO is tracked in 10 representative parcels of burnt gas, formed sequentially during combustion. Dilution of the burnt gas by unburnt air is accounted for. The model is developed for use in a machinery space simulator for training marine engineers and in a predictive emissions monitoring system. It runs in real time on a standard PC and requires experimental data for calibration. Kinetics schemes modeled include the extended Zeldovich mechanism and five schemes involving nitrous oxide. The addition of nitrous oxide reactions to the extended Zeldovich mechanism increases predicted NOx by up to 15%. The N2O reactions which give the most significant contribution to NOx in the context of a large marine diesel engine have been identified. NO from fuel bound nitrogen is likely to be significant for engines operating on residual fuel oil.