One of the main features of methane fueled spark ignition engines is their relatively slow flame propagation rates in comparison to liquid fuel applications which may lead to relatively lower power output and efficiency with increased emissions and cyclic variations. This is especially pronounced at operational equivalence ratios that are much leaner than the stoichiometric value. The addition of some hydrogen and oxygen to the methane may contribute towards speeding the combustion process and bring about significant improvements in performance and emissions. It has been suggested that the addition to the methane of products of water electrolysis generated in situ on board of a vehicle may produce such improvements.The present contribution describes the results of both experimental and analytical investigation where mixtures of hydrogen and oxygen as produced by the electrical dissociation of water were added to the intake of a spark ignition engine operating on commercial methane over a range of operating conditions. The corresponding performance when hydrogen and oxygen were added separately was also considered. It is shown that there is some improvement in engine performance with such a procedure primarily for relatively lean mixtures and low compression ratios. Such an improvement to the output will be insufficient to meet the power demand for water electrolysis should the hydrogen be produced on board through using some of the engine power output.