An experimental investigation of methane fuel oxycombustion in a variable compression ratio, spark-ignited piston engine has been carried out. Compression ratio, spark-timing, and oxygen concentration sweeps were performed to determine peak performance conditions for operation with both wet and dry exhaust gas recirculation (EGR). Results illustrate that when operating under oxycombustion conditions an optimum oxygen concentration exists at which fuel-conversion efficiency is maximized. Maximum conversion efficiency was achieved with approximately 29% oxygen by volume in the intake for wet EGR, and approximately 32.5% oxygen by volume in the intake for dry EGR. All test conditions, including air, were able to operate at the engine's maximum compression ratio of 17 to 1 without significant knock limitations. Peak fuel-conversion efficiency under oxycombustion conditions was significantly reduced relative to methane-in-air operation, with wet EGR achieving 23.6%, dry EGR achieving 24.2% and methane-in-air achieving 31.4%. The reduced fuel-conversion efficiency of oxycombustion conditions relative to air was primarily due to the reduced ratio of specific heats of the EGR working fluids relative to nitrogen (air) working fluid.