This work presents an experimental study describing a six-cylinder spark ignition engine running with a lean equivalence ratio, high compression ratio, ignition delay and used in a cogeneration system (heat and electricity production). Three types of fuels; natural gas, pure methane and methane/hydrogen blend (85% CH4 and 15% H-2 by volume), were used for comparison purposes. Each fuel has been investigated at 1500 rpm and for various engine loads fixed by electrical power output conditions. CO, CO2, HC, and NOx emissions values, and exhaust gas temperature were measured. The effect of fuel composition on engine characteristics has been studied. The results show, that the hydrogen addition increased HC emissions (around 18%), as well as performance, whilst it reduced NO. (around 31%), exhaust gas temperature, CO and CO2. (C) 2017 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.