Trace gas observations taken from the DOE G-1 aircraft are used to calculate the rate and efficiency of O₃ production in the Houston metropolitan region. Comparisons are made to the corresponding quantities calculated from data acquired during field campaigns in Phoenix and Philadelphia. Characteristics of the Houston plume are extremely high O₃ concentrations (up to 211 ppb), high O₃ production efficiencies (OPEx often greater than 10), and high O₃ production rates, sometimes exceeding 50 - 100 ppb/h. We find that comparable amounts of NO_x are processed in the 3 cities but that O₃ production efficiencies in Phoenix and Philadelphia are 25 to 50% of that observed in Houston leading to much lower peak values for O₃. Representative values for the O₃ production rate in Phoenix and Philadelphia in regions of active photochemistry are 5 and 15 ppb/h, much lower than in Houston. Calculations indicate that a high VOC to NO₂ - OH reactivity ratio is the cause of Houston's rapid and efficient O₃ production. Much of the VOC reactivity in excess of normal urban values is due to emissions of reactive olefins, in particular ethene and propene, from petrochemical facilities located in the Ship Channel region or along Galveston Bay.