Formation of the iodine monoxide radical (IO) was observed during the reaction of iodoalkyl radicals (RCHI) or cyclo-C6H10I radical with O-2 at 298 K using cavity ring-down spectroscopy, where R stands for CnH2n+l (n = 0-3). The yields of IO from the reactions of various RCH2I species with Cl atom in the presence of O-2 were determined. The energy level diagrams derived from theoretical calculations for each individual reaction support the formation of IO radicals via a direct RCHI radical oxidation pathway. (C) 2007 Elsevier B.V. All rights reserved.