This work explored the problem of using gaseous biofuels with different content of hydrogen and carbon monoxide in a three-cylinder naturally aspirated spark-ignition (SI) engine under laboratory conditions. The tests investigated the combustion process of simulated gases, whose composition was similar to the syngas generated from biomass gasification processes. During the lab tests, the engine operated at a constant speed and wide-open throttle. The engine control parameters and syngas composition influence on the combustion process, performance, as well as its harmful effects on the environment were analysed. The assay also focused on the fast-changing combustion parameters (heat release rate - HRR, mass fraction burned - MFB and combustion duration) based on incylinder pressure measurements. Gaseous fuels using during the tests were free from contamination and moisture. A clear impact of the molar fraction ratio H-2/CO in the syngas mixture on the internal parameters of the tested engine could be observed. Syngas with higher hydrogen content required the use of slightly lower ignition advance to get the maximum value of Indicated Mean Effective Pressure of the engine - IMEP. The lower heating value of the syngas fuel did not directly affect the indicated efficiency of the engine. The decisive influencing factor indeed was the type and molar fraction of combustible and inert components at the syngas mixture. For syngas mixtures with higher H-2 content, increasing the excess gas ratio increased the indicated efficiency. A clear correlation was found between the carbon monoxide content of the synthesis gas mixture and the specific emission of CO in the engine exhaust gas. Nevertheless, keeping the carbon monoxide content constant and increasing the hydrogen content in the syngas mixture resulted in a decrease of specific CO emissions. (C) 2020 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.