Application of a known hydrogen containing fuel called reformed natural gas (RNG) has been realized in a stationary combustion engine with success. The aim for this is to reduce unburned hydrogen emissions (UHC) from the engine together with an increase in efficiency.The fuel contains mainly methane, hydrogen and minor amounts of carbon dioxide. A small-scale unit for onboard production of RNG has been built in order to avoid the dependence of artificial supplementation of hydrogen. The production is carried out through means of steam reforming of natural gas. The RNG-unit together with theoretical considerations for estimating fuel composition and issues of caution are described.Theoretical studies show a potential for varying the hydrogen content between 8 and 30 vol%. Studies also show potential for remarkable increases in the methane number relative to that of the natural gas.A test engine has been fueled with RNG. The experimental results show an extension of the lean operating limit. Potential for low pollutant emissions together with an increase in efficiency for RNG fueling compared to that of conventional natural gas is seen.Recordings of the in-cylinder pressure for all operating conditions have been made. Based upon a one-zone heat-release analysis a distinct improvement of combustion quality was documented for RNG fueling.