The influence of reduction in the concentration of CO2 in biogas on performance, emissions and combustion in a constant speed spark ignition (SI) engine was studied experimentally. A lime water scrubber was used to lower carbon dioxide (CO2) levels from 41% in biogas to 30% and 20%. The tests covered the range of equivalence ratios from rich to the lean operating limit at a constant speed of 1500 rpm and at compression ratio of 13:1 with a masked valve to enhance swirl. With a reduction in the CO2 level there was a significant improvement in the performance and reduction in emissions of hydrocarbons (HC) particularly with lean mixtures. The lean limit of combustion also gets extended. Heat release rates indicated enhanced combustion rates, which are mainly responsible for the improvement in thermal efficiency. A reduction in the CO2 level by 10% seemed to be sufficient for reducing HC levels and the NO levels were also not significantly raised. The spark timings were to be retarded by about 5 degrees when the CO2 concentration was decreased by 10%. (C) 2007 Published by Elsevier Ltd.