The effect of the presence of ozone on the flammability limit and near-limit combustion of syngas (H-2/CO)/O-2 flames with N-2, CO2, and H2O dilutions was numerically studied. This study involved systematic simulations of 1-D planer laminar premixed flames under near-limit conditions using the PREMIX code considering the detailed description of chemical kinetics and molecular transport, and the intrinsic radiation as the only heat loss mechanism. The flammability limits were determined by solving the singular turning points using the one-point continuation mathematical approach. The results showed that the presence of O-3 expanded the flammability limit of syngas as O-3 reduced the lower flammability limit and increased the upper flammability limit. With increasing O-3, the H-2/CO/O2 flames could be sustained at higher N-2, CO2 and H2O dilution ratios. The laminar flame speed was also enhanced by the presence of O-3, and the enhancement effect of O-3 on the laminar flame speed was much more profound under near limit conditions, being 4-11 times greater than that under near-stoichiometric conditions. Detailed near limit flame structure and kinetic studies revealed that O-3 was consumed at relatively low temperatures (similar to 700 K), producing active radicals, such as H and OH. These radicals interfered with the H-2/CO/O-2 chemistry and thereby significantly accelerated the overall combustion process under near-limit conditions. For near-stoichiometric flames, the large disparity between the characteristic temperatures of O-3 related reactions (similar to 500 K) and those of H-2/CO/O2 chemistry (>1250 K) depressed the interaction between O-3 related reactions and H-2/CO/O2 chemistry and thus only limited O-3 enhancement effect was observed under near-stoichiometric conditions. (C) 2016 Elsevier Ltd. All rights reserved.