The Department of Energy (DOE) G-1 research aircraft made flights on 13 days during the month long Texas 2000 Air Quality Study (TexAQS 2000) to understand the sources and formation mechanism of the very high concentration O3 plumes that are frequently observed in the Houston metropolitan area during the late summer. On six of those days the aircraft sampled plumes exhibiting O3 concentrations in excess of 150 ppbv at a number of different locations in and around the greater Houston area. The composition of these plumes differed significantly from those typically observed in other urban areas, exhibiting unusually high concentrations of hydrocarbon oxidation products such as HCHO and photochemical product species such as peroxides. Estimates of the integrated formation efficiency of O3 with respect to NOx, OPEx, indicate that O3 had been formed in these high concentration plumes with efficiencies ranging between 6.4 and 11 ppbv O3 per ppbv of NOx consumed. Without exception, back trajectories from the locations where these high O3 plumes were observed passed over, or in close proximity to, sources of NOx and hydrocarbons surrounding the Houston Ship Channel. Calculations of instantaneous ozone formation rates and efficiencies using a box model constrained by measurements of stable species showed that ozone formation over and around the Houston Ship Channel could be very rapid (instantaneous rates up to 140 ppbv/h) and very efficient (OPEx up to 28) and, in some instances, limited by the availability of NOx. High concentrations of reactive hydrocarbons and NOx emitted by industries in this area appeared to be the cause of these high rates and efficiencies. Examination of the distribution of photochemical product distributions in the high O3 plumes arising from Ship Channel emissions suggests that O3 formation in these plumes was much more NOx limited than in typical urban plumes at equivalent times in their evolution. However, chemical/transport model simulations with realistic emissions inventories are needed to resolve the question of whether a NOx- or hydrocarbon-based control strategy would be most effective at controlling greater Houston O3 concentrations.