Phototransformations of halomethanes containing chlorine and bromine (CCl3Br, CHCl2Br, CH2ClBr) or bromine and fluorine (CBr3F) have been investigated under aerobic and anaerobic conditions both in homogeneous system and heterogeneous photocatalysis. For all of those compounds, the complete disappearance of the primary compound and the stoichiometric concentration of halides was achieved. Several halogenated intermediates and oxygenated compounds were identified, so that it was possible to predict the degradation pathways followed by such halomethanes. Whereas the reductive steps are predominant in the initial degradation of CCl3Br and CBr3F, the oxidative steps are predominant in the initial CH2ClBr steps. The two pathways have comparable importance for CHCl2Br degradation. Methanol, acting as a hole scavenger, strongly increases the rate of disappearance for CCl3Br and CBr3F. (C) 2001 Elsevier Science B.V. All rights reserved.