Dedicated exhaust gas recirculation (D-EGR) is to generate H-2 via fuel-rich combustion and viewed as a potential technique to meet future emission regulations without further after-treatment. In this study, firstly, the H-2/CO formation through fuel-rich combustion in a single-cylinder natural gas spark ignition engine was quantitatively characterized by gas chromatography. Then, the effect of H-2/CO addition on stoichiometric natural gas combustion performance and emission characteristics at 15% and 20% EGR levels was investigated. Finally, reaction path analysis and the brute-force sensitivity of ignition delay were conducted to evaluate the effect of H-2 addition on reaction process. The yields of H-2 and CO approximately linearly increased from similar to 2% to similar to 10% as the equivalence ratio varied from 1.1 to 1.5. The H-2/CO addition accelerated the flame speed of mixture and significantly shortened the combustion duration, significantly improving the indicated thermal efficiency and the total unburned hydrocarbon with the acceptable penalty of increased NOx and CO emissions. Numerical results revealed that OH + H-2 = H + H2O and H + O-2 = O + OH were the most sensitive reactions with the presence of H-2. This study delivered a quantitative basis for the optimization of D-EGR fueling strategies in natural gas engines. (C) 2017 Elsevier Ltd. All rights reserved.