Ozone formation in the Houston area during a period of high ozone concentrations that occurred on 29 August 2000 during the TexAQS 2000 study is examined to understand differences in the sources of O₃ precursors and the rate and efficiency of ozone formation over the city of Houston and the industrialized Ship Channel region to the east of Houston. From late morning through late afternoon on 29 August, a period of stagnation occurred, allowing accumulation of O₃ and product species separately over downtown Houston and the Houston Ship Channel. Three aircraft flights were made in the region, starting from about 0900 CST and extending to about 1700 CST. A localized plume of high O₃ ranging between 120 and 200 ppb was observed over the Ship Channel on all of these aircraft flights. Over the same time period, O₃ concentrations over the city were much lower ranging between 40 and 90 ppb. NO _x concentrations measured in the two regions in the late morning were roughly the same, but hydrocarbon reactivities over the industrial area were much higher, by as much as a factor of 10. Photochemical box model calculations constrained by observations of NO _x , hydrocarbons, O₃, and other stable species indicated that the instantaneous ozone formation rate was much lower (3–18 ppb/h) over downtown Houston than it was over the Ship Channel (3–80 ppb/h). The much faster O₃ formation rates and higher concentrations observed over the Ship Channel are attributed to the much higher hydrocarbon reactivity, the majority of which was contributed by low molecular weight alkenes. These high hydrocarbon reactivities also caused O₃ over the Ship Channel to be produced with much higher efficiency than over urban Houston. Comparison of photochemical product distributions suggests that O₃ formation in the urban area is much more hydrocarbon limited than in the Ship Channel, consistent with the geographic distribution of major hydrocarbon sources in the area.