Reactive sorption of two chlorinated volatile organic compounds (CVOCs) has been investigated on ZSM-5 zeolites using in situ FTIR. Zeolites exchanged with different cations (Cr-ZSM-5, Ag-ZSM-5, and H-ZSM-5) and with atomic Si/Al ratios of 15, 25, and 40 were examined. Two dilute vapor phase CVOCs, trichloroethylene (TCE) and 1,3-dichloropropene (DCP) were added in N-2 at both 25 and 120 degreesC to the zeolite samples in an FTIR transmission cell during separate experiments. For TCE, based on repetitive FTIR scans, a reaction mechanism that included formation of dichloroacetaldehyde and phosgene was proposed for the ambient data. Results show that chemisorbed oxygen, coordinated with the active catalytic sites, formed C=O bonds with TCE under the oxygen depletive cell conditions studied. A dehydrochlorination mechanism was proposed to explain the ambient DCP data. Relative ambient reactivity by cation type for both CVOCs was Cr > H > Ag. This ordering is consistent with the oxygen pick-up capacities of the exchanged cations. Total acidity values of the catalysts and their relative site strengths appear to be of secondary influence. No clear trends could be found relating reactivity to Si/Al ratio, presumably because the range of values studied was too narrow. For both feeds and all catalysts, coke formation was the main reaction at 120 degreesC. (C) 2004 Elsevier B.V. All rights reserved.