This study reports the role of the homogeneous photochemical oxidation (PCHO) reaction occurring in the gas-phase during heterogeneous photocatalytic degradation (PCTD) reaction of trichloroethylene (TCE) in humid and dry atmospheres. PCTD reaction was carried out in the presence of TiO2 glass fiber cloth (TiO2-GFC) while chlorine radicals generated from the exposure of chlorine gas to UV-illumination induced the PCHO reaction. The reaction profiles of the PCHO and PCTD reactions monitored independently in dry and humid atmospheres by in situ FTIR spectroscopy reveal the difference between the reaction pathways of phosgene formation and DCAC oxidation. Degradation of TCE to CO2 increased from 52% at 0% relative humidity (RH) to 71% at 100% RH during PCTD reactions while during PCHO reaction it decreased from 27% at 0% RH to 2% at 100% RH. Reactivity of both PCHO and PCTD reactions decreased with increasing RH. RH corresponding to similar to25% RH was found to be the most favorable condition for PCTD reactions because reasonably high reactivity and mineralization of TCE can be accomplished simultaneously. (C) 2004 Elsevier B.V. All rights reserved.