Though hydrogen fueled spark ignition engine can operate at high thermal efficiency with almost zero emission of HC and CO, the high level of NOx poses problems. The high combustion temperature and lean mixtures used are the reasons. In this work, the effect of N-2, CO2 and hot EGR gas as diluents in the intake charge to suppress NO emission in a manifold injected hydrogen fueled SI engine was studied. Nitrogen as a diluent is not so effective at low loads while inducting smaller amounts, but very effective at higher loads where the mixture becomes richer and the dilution effect (oxygen depletion) is significant. On other hand, carbon dioxide is a good diluent with relatively better thermal effect and diluent effect and effectively controls NO emission at all output regions. However this is at the expense of thermal efficiency. Recirculating hot exhaust gas which contains both N-2 and steam comes in between N2 and CO2 in terms of its effectiveness. On the whole N-2 is the most effective as it has minimum impact on thermal efficiency for a given level of permissible NO emission. Thus it is felt that cold EGR could be a good option. In all cases, a good control system is necessary to supply correct quantity of diluent. (c) 2006 International Association for Hydrogen Energy. Published by Elsevier Ltd. All rights reserved.