An experimental investigation of the thermal efficiency, combustion efficiency, and CoV IMEP, of methane fuel oxycom-bustion in an SI engine has been carried out. Compression ratio, spark-timing, and oxygen concentration were all varied. A variable compression ratio SI engine was operated on both wet and dry EGR working fluids, with results illustrating that the efficiency of the engine operating with a large amount of EGR was significantly reduced relative to methane-in-air operation over all oxygen concentrations and compression ratios. The maximum thermal efficiency of wet EGR, dry EGR, and air was found to be 23.6%, 24.2%, and 31.4%, respectively, corresponding to oxygen volume fractions of 29.3%, 32.7% and 21%. Combustion efficiency was above 98% for wet EGR and approximately 96% for dry EGR. CoV IMEP was low for both cases. The much lower efficiency of both EGR cases relative to air is primarily a result of the reduced specific-heat ratio of the EGR working fluids relative to air working fluid.