Different CH4 and CO2 mixtures were used as fuel in this work; they were fed into a spark-ignited engine equipped with devices allowing spark advance, gas delivery and gas consumption to be measured. Engine bench-tests revealed changes in the main operation parameters and emissions. The results showed that increasing CO2 percentage in the mixture increased the spark angle, reduced maximum power and torque and reduced exhaust emissions (by 90% in some cases) when DAMA resolution 1015/2005 was applied. The main components to be considered when an engine of this type operates with gas fuel were also recognised.