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, CH4, C2H4 and C3H8. Enhancement of S-L due to O-3 addition was consistently observed for CH4 and C3H8 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 CH4/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, C2H4 in this study. With O-3 addition, C2H4/air S-L decreased at room temperature and pressure, owing to the heat loss induced by the exothermic ozonolysis reaction between O-3 and C2H4 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. (C) 2015 The Combustion Institute. Published by Elsevier Inc. All rights reserved.