Three identical mobile chambers were designed and constructed for use in outdoor and field investigations. These chambers were employed during the Texas Air Quality Study (TEXAQS 2000) in Houston, Texas. Experiments were performed to investigate the ability of chlorine atoms (Cl¿) to enhance ozone formation in captive ambient air from the Houston, Texas, area under the following conditions: (1) Release of Cl¿ precursor into captive ambient air, (2) release of Cl simultaneously with other ozone precursors (i.e., reactive hydrocarbons and nitrogen oxides) into captive ambient air, and (3) release of Cl into captive aged air masses. The investigation yielded the following results: (1) Chlorine atoms can enhance ozone formation in Houston's ambient air, (2) Cl¿ provide the greatest enhancement to reactivity in the morning hours after sunrise, (3) the enhancement by Cl¿ in the morning hours is greater than comparable amounts of reactive hydrocarbons such as propene, and (4) Houston air masses can display much variability in reactivity. This paper describes some of the unique characteristics of the chambers employed during this study and results from captive air experiments performed using these environmental chambers during TEXAQS 2000.