An instrumented aircraft was used to study anthropogenic emissions and subsequent ozone and reactive nitrogen photochemistry in the continental boundary layer downwind of Houston, Texas. Measurements of ozone, carbon monoxide, NOx, and NOx oxidation products were conducted from the NOAA WP-3 aircraft during the 2006 Texas Air Quality Study under a variety of meteorological conditions. Sixty-five crosswind transects of plumes from Houston urban and industrial areas performed on 10 daytime flights from 13 September to 6 October 2006 are examined. Coincident measurements of NOx and its oxidation products show that NOx was oxidized predominately to nitric acid and peroxy acyl nitrates on time scales of a few hours. The observed relationships between O3 and NOx oxidation products are affected by both photochemistry and mixing of different air masses. On four flights, background pollutant mixing ratios were constant and CO to NOy enhancement ratios in downwind plume transects remained at the emission ratio. The enhancement ratio of O3 to NOx oxidation products was also nearly constant and could be used to derive ozone production efficiency (OPE) in plumes downwind from the Houston area. On the other flights, variable mixing of regionally polluted background air with plumes caused CO to NOy and O3 to NOy − NOx enhancement ratios to increase as plumes were transported. In such cases, enhancement ratios do not solely reflect plume processing, and OPE could not be determined. The OPE averages 5.9 ± 1.2 in coalesced plumes from urban and petrochemical industrial sources in Houston, with higher values in isolated plumes downwind from petrochemical facilities located along the Houston ship channel.