The effect of O 2 composition modulation on the dynamics and structure of lean CH 4 /air premixed flames was computationally and experimentally investigated, in light of its relevance to oxygen-enhanced combustion (OEC). OEC can result in conditions of stable combustion with reduced NO x emissions. Experiments were conducted in the stagnation flow configuration. Laminar flame speeds, as well as temperature and NO x concentration profiles, were determined. The laminar flame speeds were measured by using laser Doppler velocimetry. Temperatures were measured by using thermocouples, and the NO x profiles were measured by using a chemiluminescence analyzer. The experiments were modeled in detail by using the GRI 3.0 mechanism, and satisfactory agreement was found between the experimental and simulated results, for both laminar flame speeds and nitrogen chemistry. It was found that addition of extra O 2 in the oxidizer noticeably extends the lean flammability limit. Thus, stable combustion can be achieved at leaner conditions, which are also characterized by reduced fuel consumption, lower flame temperatures, and reduced NO x emissions. The reduction of NO x is caused by the synergistic effect of temperature and N 2 concentration reductions as more O 2 is added. Furthermore, studies conducted by extensively modulating the O 2 composition in the oxidizer, and by keeping the ratio of N 2 to CH 4 supplies constant, revealed non-monotonic effects on both the laminar flame speed and the maximum NO x concentration. The nonmonotonic effect on the laminar flame speed was found to be a result of the competing effects of O 2 concentration and temperature variations on the main branching H β«Χβ¬ O 2 β OH β«Χβ¬ O reaction, which critically affects propagation. The non-monotonic effect on the maximum NO x concentration was explained from first principles by assessing the relative importance of the reactions that are largely responsible for NO production and destruction.