Experiments have been conducted on a gas fueled spark ignition engine using natural gas and two hydrogen containing fuels. The hydrogen containing fuels are Reformulated Natural Gas (RNG) and a mixture of 50% (Vol.) natural gas and 50% (Vol.) producer gas. The producer gas is a synthetic gas with the same composition as a gas produced by gasification of biomass.The hydrocarbon emission, measured as the percentage of hydrocarbons in the fuel, which passes unburned through the engine, was for the mixture of natural gas and producer gas up to 50% lower than the UHC emissions using natural gas as fuel. The UHC emission from the experiments using reformulated natural gas was 15% lower at lean conditions. Furthermore, both hydrogen-containing fuels have a leaner lean burn limit than natural gas.The combustion processes from the experiments have been analyzed using a three-zone heat release model, which is taking the effect of crevices into account. The analysis showed that the two hydrogen containing fuels cause an increase in the turbulent flame speed resulting in faster combustion compared to natural gas. Even though both hydrogen containing fuels have a lower adiabatic flame temperature the increased turbulent flame speed causes a higher maximum temperature of the burned gas compared to natural gas. It is this increased maximum burned gas temperature which causes a higher NOx emission from the hydrogen containing fuels.