Abstract The effect of ozone (O 3 ) addition on laminar flame speeds ( S L ) across a wide pressure range was investigated experimentally and numerically using three fuels, CH 4 , C 2 H 4 and C 3 H 8 . Enhancement of S L due to O 3 addition was consistently observed for CH 4 and C 3 H 8 mixtures over a range of lean to rich equivalence ratios, based on comparisons of S L measured with and without O 3 addition. For both fuels, simulation results agree with experimental results, with the best predictions at near stoichiometric conditions and the largest discrepancies for fuel-rich cases. A significant increase in the S L enhancement was observed at elevated pressures: the enhancement in the measured S L for a stoichiometric CH 4 /air mixture with 6334 parts per million (ppm) O 3 addition increased from 7.7% at atmospheric pressure to 11% at 2.5 atm. Elevated pressure both promotes O 3 decomposition, which provides O atoms, and suppresses diffusion of H, which reduces the influence of the O 3 +H=OH+O 2 reaction. Together, these lead to the increased S L enhancement with pressure. In contrast to the results for the two saturated hydrocarbons, both detrimental and beneficial effects due to O 3 addition were observed for the unsaturated hydrocarbon fuel, C 2 H 4 in this study. With O 3 addition, C 2 H 4 /air S L decreased at room temperature and pressure, owing to the heat loss induced by the exothermic ozonolysis reaction between O 3 and C 2 H 4 in the mixing process, but increased as the ozonolysis reactions were minimized when reactants were cooled to 200 K or pressure was decreased below 0.66 atm. These experimental results were successfully explained by a numerical model that includes a new ozonolysis sub-mechanism.