This paper describes an investigation into the combustion of simulated biogas in a spark ignition engine. The biogas has been simulated by the mixing of methane with carbon dioxide to create a range of biogas compositions. The ratio of methane to air has been maintained at a constant throughout the study. The quantity of carbon dioxide was increased until the engine stalled at a concentration of 35%. The engine employed in the investigation was a 1.3 litre Rover ‘A’ series unit which had four cylinders, overhead valves, a compression ratio of 9.5:1 and was run at a fixed speed of 1500 rev/min. The in-cylinder pressure signals was recorded using a high speed data acquisition system and was then used as an input into a two zone combustion model to calculate the mass fraction burnt. The model was adapted to the combustion chamber geometry of the engine under study which enabled the turbulent burning velocity values to be obtained. The present study showed a sharp decrease in the turbulent burning velocity as the concentration of carbon dioxide increased in the biogas mixture, as expected. A reduction of 50% in the turbulent burning velocity was observed with a 70% methane: 30% carbon dioxide ratio.