The potential energy surfaces for the reaction of furanyl and oxazolyl radicals with O-2 have been examined using the B3LYP method. The initial production of the arylperoxy radical followed by either simple decomposition or rearrangement to yield several intermediates (aryloxy, dioxiranylaryl, or dioxetanylaryl radicals) has been explored. Transition state structures for most of the steps are presented as well as relative free energies over a range of temperatures from 298 to 2000 K. The energetics of the analogous intermediates for the reaction of O-2 and other five-membered heterocyclic radicals derived from pyrrole and thiophene are also provided. The loss of an O atom is generally the most accessible and energetically favored pathway of decomposition at all temperatures. Dioxiranyl formation is favored over O-2 loss at temperatures less than or equal to 500 K and favored in the same temperature range over O atom loss in several cases. Dioxetanyl formation incurs the greatest barrier to formation, and direct routes are not available in every molecule surveyed. However, in some cases the dioxetane radicals transform rapidly into very stable species.