A chemical kinetic model is used to study the combustion characteristics of hydrogen under conditions found in internal combustion engines. Laminar flame speeds are predicted under the high-pressure and high-temperature conditions found in a spark-ignition engine. The variation of laminar flame speed with temperature, pressure, and equivalence ratio is examined. The autoignition of hydrogen and hydrogen-natural gas mixtures is also studied under conditions found in a free-piston compression-ignition engine. This engine is expected to exhibit high efficiencies operating on hydrogen, The results suggest that adding small amounts of natural gas to hydrogen will reduce NOx emissions from a free-piston compression-ignition engine. Calculations showed that the predicted NO concentrations correlated well with the maximum temperature in the cycle and that the NO was mostly produced by Ng + O = NO + N.