An experimental evaluation has been made of the power output, specific fuel consumption and thermal efficiency of a modern 4 cylinder spark ignition engine operating on gasoline and natural gas. Tests have been conducted at various speeds, spark advance angles and air-fuel ratios, all at wide open throttle. Performance on gasoline and natural gas has been compared and optimum spark advance determined for natural gas operation. With natural gas operation, brake power was found to decrease by between 11.3% and 16.6% compared to gasoline operation. Brake specific fuel consumption was found to decrease with natural gas operation, although this is due to the higher calorific value of natural gas. In terms of brake thermal efficiency, operation with natural gas was found to be less efficient than with gasoline. This is due to the lower flame speed of natural gas and the proportionally higher friction losses at the reduced power levels.