Uncertainty of fuel supply in the energy sector and environmental protection concerns have motivated studies on clean and renewable alternative fuels for vehicles as well as stationary applications. In recent years, many efforts have been put on developing alternative fuels for internal combustion engines. Among all fuel candidates, hydrogen is generally believed to be a promising alternative, with significant potential for a wide range of operating conditions. Different from fossil fuels, it can be produced through water electrolysis, fuel reforming and biological hydrogen production. Currently, methane is the gaseous fuel used most often, in spark ignition (SI) as well as dual fuel configurations. Addition of H2 to CH4 can improve the already good qualities of this fuel, and compensate its weak points, such as low laminar flame speed. Syngas is essentially a mixture of hydrogen, carbon monoxide and higher-order hydrocarbon gases, mainly methane. It can be generated through biomass gasification, via reactions that involve natural gas and coal, as well as the recycling of refinery byproducts. For this reason syngas is considered as a strong candidate to replace many fuels currently in use. In this paper, a comparison was carried out between CH4, two CH4/H2 blends and two mixtures of CO and H2, the last one taken as a reference composition representative of syngas. It is imperative to fully understand and characterize how these fuels behave in various conditions. In particular, a deep knowledge of how hydrogen concentrations affect the combustion process is necessary, given that it represents a fundamental issue for the optimization of internal combustion engines. To this aim, flame morphology and combustion stability were studied in a SI engine under lean burn conditions. The engine was fuelled with CH4, CH4/H2 (75-25%vol and 50-50%vol) and H2/CO (50-50%vol and 75-25%vol). The engine was operated at fixed rotational speed at wide open throttle. Lean operation was studied in detail through combined methodologies based on thermodynamic analysis and optical diagnostic. Specifically, cycle resolved digital imaging was applied to follow flame front propagation. Image processing was applied to evaluate flame speed and other morphology parameters, including flame displacement and centroid motion. Moreover, a detailed study of local parameters such as wrinkling was presented. The excess air ratio was raised from 1.4 to values close to the flammability limit for each fuel. In order to maintain roughly the same fluid dynamic conditions (swirl, tumble, turbulence intensity, etc.) spark timing was set according to the maximum brake torque of the baseline case (CH4 ) in the condition of lambda (λ) 1.4.