The oxidation of laminar premixed natural gas flames has been studied experimentally and computationally with variable mole fractions of hydrogen (0, 20, and 60%) present in the fuel mixture. All flames were operated at low pressure (0.079 arm) and at variable overall equivalence ratios (0.74 < phi < 1.0) with constant cold gas velocity. At the same global equivalence ratio, there is no significant effect of the replacement of natural gas by 20% of H-2. The small differences recorded for the intermediate species and combustion products are directly due to the decrease of the amount of initial carbon. However, in 60% H-2 flame, the reduction of hydrocarbon species is due both to kinetic effects and to the decrease of initial carbon mole fraction. The investigation of natural gas and natural gas/hydrogen flames at similar C/O enabled identification of the real effects of hydrogen. It was shown that the presence of hydrogen under lean conditions activated the H-abstraction reactions with H atoms rather than OH and O, as is customary in rich flames of neat hydrocarbons. It was also demonstrated that the presence of H-2 favors CO formation. (C) 2008 The Combustion Institute. Published by Elsevier Inc. All rights reserved.