Chlorine atoms have been implicated in tropospheric ozone formation in certain urban areas, such as Houston, Texas. To determine the presence and extent of Cl chemistry in the Houston area, we measured specific tracers from isoprene oxidation by Cl. Isoprene is a highly reactive nonmethane hydrocarbon (NMHC) that is emitted by vegetation, and to a lesser degree, by anthropogenic activities. We investigated the isoprene oxidation process under ambient conditions at the LaPorte airport experimental site, and were able to characterize the different oxidation processes that were occurring. Our results confirm that Cl is indeed an oxidant in the Houston urban area. Calculations show that episodic concentrations of Cl can approach 2-4 x 10⁵ Cl atoms cm^-3. The extent to which Cl serves as the major contributor to the persistent ozone problems in Houston is yet to be determined.