In situ hydrogen production is carried out by a catalytic reformer kit set up into exhaust gases for a CHP plant based on spark ignition engine running under lean conditions. An overall auto-thermal reforming process is achieved. Hydrogen production is mainly dependent on O-2 content in exhaust gases. Experiments are conducted at constant speed at 2 air/fuel ratios and 4 additional natural gas flow rates. H-2 content varies in the range 6-10% in vol. H-2 content effect is analysed with respect to performance and emissions. Comparing with EGR shows an increasing of electrical efficiency of 1% whilst heat recovery decreases by 1%. NO and HC decrease by 18% and 12%, but CO increases by 14%, respectively. The results show that: (i) graphite joints were destroyed under effect of H-2 and high temperature; (ii) a cold spot appeared in the RGR line, and condensation has as consequence a carbon deposit; and (iii) no back-fire or knock occurred. Copyright (C) 2010, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All] rights reserved.