Results are presented of tests with a variable compression ratio Ricardo E6 single-cylinder spark-ignition engine operating on simulated biogas formed from different mixtures of domestic natural gas and carbon dioxide. The fraction of carbon dioxide in the simulated biogas was changed from 0 to about 40% by volume to cover the range typically encountered in sources of biogas in practice. The tests covered a range of air:fuel ratios from rich to the lean operating limit at four speeds and a number of compression ratios. Measured results are given for power, exhaust temperature, thermal efficiency and the mole fractions of the regulated emissions carbon monoxide (CO), oxides of nitrogen (NOx) and total unburnt hydro-carbon (HC) in the exhaust gases. Experimental results indicate that the presence of carbon dioxide can improve NOx emissions, but since lower cylinder pressures result, engine power and thermal efficiency are reduced and the level of unburnt HC is increased. Measured data, however, suggest that it is possible to significantly increase the compression ratio as an effective means of improving biogas-fuelled engine performance when CO2 is present. While this would normally raise the emissions of both NOx and HC, the former is offset by the CO2 content of the biogas.