The effects of chlorine radical generation during the photocatalytic oxidation of aromatic contaminants in air can be significant. Previous studies have shown that cofeeding chlorinated olefins (TCE, PCE) may increase the removal of branched aromatics (toluene, xylenes), presumably by initiating chlorine radical chain reactions. Hydrochloric acid catalyst pretreatments produce a similar enhancement of aromatic conversions. However, both approaches were ineffective for rate enhancement toward benzene, while hydrobromic and hydriodic acid pretreatments diminished the photocatalytic oxidation of both benzene and toluene. The present study broadens this halide acid inquiry to include hydrofluoric acid catalyst pretreatments and to examine halide acid influences on the removal of m-xylene as well. Only HCl pretreatments enhanced activity, and only for the branched aromatics (toluene and m-xylene). These results appear consistent with a thermodynamic analysis proposed here, which indicates that (1) fluorine radical generation is not energetically feasible under the conditions considered, (2) bromine and iodine radical generation is energetically feasible, but the radicals are insufficiently reactive to initiate the degradation of aromatics, and (3) chlorine radicals are predicted to be sufficiently energetic to abstract hydrogen from the methyl groups of branched aromatics, but not from the more strongly bound hydrogen atoms on the aromatic ring. These calculations are all in accord with our experimental results for the photocatalytic oxidation aromatics. The consistency of these thermodynamic arguments is also further support for the presumed halide radical pathway for rate enhancement. (C) 2003 Published by Elsevier Science (USA).