A study is presented of the influence of using mixtures of natural gas and hydrogen in different fractions (0, 25, 50, 75, 100%) on the combustion velocity and cycle-to-cycle variations in a spark ignition engine. The experimental facility consists of a single-cylinder spark ignition engine. The engine rotational speeds are 1000, 1750 and 2500 rpm. Fuel/air equivalence ratio was kept constant equal to 0.7 during the experiments. A two-zone thermodynamic combustion diagnosis model, based on solving the mass and energy conservation equations, is used to analyze the experimentally obtained pressure combustion chamber in the engine. The two-zone model considers a spherical flame front centred at the spark plug, and solves the intersection of the flame front with the piston, cylinder head and cylinder wall, in order to provide the values of the flame radius corresponding to the burned mass volume and the surfaces for heat to the piston and walls. An automatic procedure based on genetic algorithms is used to determine the optimum parameters needed for combustion diagnosis: Angular positioning and pressure offset of the pressure register, dynamic compression ratio, and heat transfer coefficients. The paper focuses on using the values of the burning velocity computed from the pressure register and especially on the analysis of the cycle to cycle variation in the natural gas/hydrogen fuelled engine, quantified through the standard deviation and the coefficient of variation of the burning speed. Increasing the hydrogen content in the mixture with natural gas increases its burning velocity. This effect is linear with hydrogen fraction, except for very high values of the fraction, when the effect of hydrogen dominates combustion. Additionally, and of practical importance, increasing the hydrogen fraction reduces the relative dispersion of combustion. This effect of hydrogen addition on reducing combustion variability is evident from 25% hydrogen content. Copyright (C) 2015, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.