A single-cylinder variable-compression ratio spark-ignition engine was used to compare the performance and emissions for gasoline and natural gas with values for simulated biogas containing different proportions of carbon dioxide. The gaseous fuels had leaner misfire limits than the gasoline. Brake power was lower even at a higher compression ratio compared with gasoline operation at the normal value. Brake thermal efficiency was less than with gasoline at the same compression ratio and decreased with increasing carbon dioxide proportion. At the higher compression ratio and moderate speeds, the brake thermal efficiency for all the gaseous fuels was higher than for the baseline gasoline. The gaseous fuels gave less total unburnt hydrocarbon and carbon monoxide. Higher nitrogen oxide emissions were produced with natural gas at the higher compression ratio than with normal gasoline operation. The presence of carbon dioxide in the biogas, however, counteracted this effect and nitrogen oxides levels were lowest for these fuels.