Injecting liquid water into a fuel/air charge is a means to reduce NO x emissions. Such strategies are particularly important to hydrogen internal combustion engines, as engine performance (e.g., maximum load) can be limited by regulatory limits on NO x . Experiments were conducted in this study to quantify the effects of direct injection of water into the combustion chamber of a port-fueled, hydrogen IC engine. The effects of DI water injection on NO x emissions, load, and engine efficiency were determined for a broad range of water injection timing. The amount of water injected was varied, and the results were compared with baseline data where no water injection was used. Water injection was a very effective means to reduce NO x emissions. Direct injection of water into the cylinder reduced NO x emissions by 95% with an 8% fuel consumption penalty, and NO x emissions were reduced by 85% without any fuel consumption penalty.