The introduction of alternative fuels is crucial to limit greenhouse gases. CNG is regarded as one of the most promising clean fuels given its worldwide availability, its low price and its intrinsic properties (high knocking resistance, low carbon content...). One way to optimize dedicated natural gas engines is to improve the CNG slow burning velocity compared to gasoline fuel and allow lean burn combustion mode. Besides optimization of the combustion chamber design, hydrogen addition to CNG is a promising solution to boost the combustion thanks to its fast burning rate, its wide flammability limits and its low quenching gap.This paper presents an investigation of different methane/hydrogen blends between 0% and 40 vol. % hydrogen ratio for three different combustion modes: stoichiometric, lean-burn and stoichiometric with EGR. The main objectives are to identify the complex mechanisms involved in the combustion process and to define the optimal hydrogen ratio for each combustion mode.The study combines engine tests and 0D modeling. Tests were carried out on a spark-ignited single-cylinder engine adapted to CNG operation with 2 different compression ratios 9,5:1 and 11,5:1. Computations allow studying separately the different phenomena linked to the progressive addition of hydrogen in the fuel. Hence, using 0D modeling, the effects due only to the combustion speed evolution, as a function of hydrogen ratio in the fuel, were then quantified using experimental results as comparison basis.The engine test results reveal that the impact of hydrogen is limited in stoichiometric conditions except for CO₂ savings. A ratio of 10 to 20 vol. % of hydrogen seems to be optimal to reach interesting HC emissions reduction without an excessive lowering of the operation range. The results are more encouraging for lean-burn operation as the lean limit of equivalence ratio is extended for more than 0,125 with 40 vol. % of hydrogen. NOx emissions and consumption were significantly reduced (-92% for NOx) while maintaining constant HC and CO emissions compared to methane operation. Hydrogen stabilizes and speeds up combustion especially in very diluted mixtures. Furthermore, hydrogen increases EGR tolerance for the same reasons as in lean-burn mode: in comparison, identical very-low NOx levels were reached with smaller consumption gains. Finally, the engine test results show that the highest tested ratio of hydrogen (up to 40%) in methane for very diluted mixtures with air or EGR drastically reduce NOx emissions without penalty on engine performances.Computations prove that the evolution in terms of thermodynamic properties of the fuel is not the most important contribution to the evolution of combustion. The effect of hydrogen addition contributes mainly to increase the combustion speed. Moreover, lean burn operation results demonstrate that the increase of combustion speed due to hydrogen addition is enhanced. This can only be explained by the increase of thermo-diffusivity which has an opposite and larger effect than the slowdown of laminar flame velocity conventionally described in the literature.