Steady state experiments on a single cylinder spark ignition engine were performed to investigate the effects of compression ratio on nitric oxide and hydrocarbon emissions using natural gas as the fuel. Compression ratios between 8 and 15 were investigated. Constant throttle tests were performed at different equivalence ratios, throttle openings and spark timing settings covering a wide range of these parameters.In general, nitric oxide and hydrocarbon emissions were found to increase with compression ratio at fixed spark timing. With optimised (MBT) spark timing, however, reductions of emissions could be achieved at high compression ratio. This indicates that a fully optimised natural gas fuelled engine may be able to achieve high efficiency and low emissions.