In this work a methodology is presented for studying the influence of the addition of small percentages of hydrogen in spark-ignition engines running on natural gas, on CO and the NOx exhaust emissions. Thermo-chemical models are used to predict the exhaust emissions. The general inputs of these models are the geometry of the combustion chamber, correlations for the laminar burning velocity of the fuels considered and an expression of the flame speed ratio (ratio of turbulent speed to laminar speed). In the present work, the geometry of the combustion chamber and the flame speed ratio of the engine are first estimated from data corresponding to similar engines, since a direct measurement of them was not possible. In a second step, correlations for the burning velocity of natural gas and hydrogen mixtures as a function of the temperature are determined inside a constant volume combustion bomb designed for the study of pre-mixed combustions. In a third step, a quasi-dimensional model is used to predict the thermal parameters evolution and the optimum ignition timing in different conditions, and then a chemical-kinetic model is used to predict the NOx and CO exhaust emissions of the spark-ignition engine by using the results obtained in the previous sections as inputs. (C) 2010 Professor T. Nejat Veziroglu. Published by Elsevier Ltd. All rights reserved.