The combustion process of a four-stroke optically accessible single cylinder Port Fuel Injection spark ignition (PFI SI) engine was experimentally investigated. It was fueled with two methane/hydrogen blends. The in-cylinder pressure and the related data were analyzed as indicators of the combustion quality. 2D-digital imaging measurements were performed to evaluate the flame propagation. UV visible spectroscopy allows to characterize the combustion by means of the detection of OH* and CH*. The exhaust was characterized using conventional analyzers. For the methane/hydrogen blends the indicated data suggests an increase of the thermal efficiency and a decrease of the combustion duration with the increase of the hydrogen fraction. The optical results highlight a more homogeneous mixture that increases the combustion reaction rate and provides a more uniform and rapid flame propagation. On the other hand, high NO emissions were measured likely because of the higher combustion temperature due to hydrogen addition. Copyright (C) 2014, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.