Petrochemical industrial sources can emit large amounts of highly reactive hydrocarbons and NOx to the atmosphere; in the summertime, such co-located emissions are shown to consistently result in rapid and efficient ozone formation downwind. Airborne measurements show hydrocarbon reactivity in petrochemical source plumes in the Houston, TX metropolitan area is primarily due to emissions of the alkenes propene and ethene; depending on the source and its location, isoprene also contributed substantially. Ozone formation rates and yields per NOx molecule oxidized from petrochemical industrial sources were significantly higher than rates and yields observed in urban or rural power plant plumes. These observations suggest that reductions in reactive alkene emissions from petrochemical industrial sources coupled with simultaneous reductions in NOx would be required to most effectively reduce ozone in the Houston metropolitan area.