The potential energy surface for the unimolecular decomposition of phenylperoxy radical has been explored using the B3LYP method. Several pathways were considered including the initial formation of the phenoxy. dioxiranyl, 1,2-dioxetanyl, 1,3-peroxy, and p-phenylquinone radicals. Transition states for all pathways on the potential energy surface are presented. Ar all temperatures studied (T less than or equal to 1250 K), the energetically most favored pathway is the dioxiranyl pathway which leads to the formation of cyclopentadienyl radical and CO2, pyranyl radical and CO: or an acyclic C6H5O2 radical structure as products. The ring-opening reactions are very competitive with formation of CO and CO2 as products.