in an attempt to clarify the fate of ethene in the cleaner regions of the troposphere, where the NO, concentrations are low, the energetical viability of four possible pathways for the unimolecular decomposition of beta-hydroxyethylperoxy radical (1) have been investigated from the theoretical point of view. The direct hydrogen atom transfer from 9beta-hydroxyethyl radical to triplet O-2 giving either vinyl alcohol or oxirane has also been investigated. Each pathway has been characterized by means of density functional theory (B3LYP) and quantum-mechanical (CCSD(T)) calculations with basis sets ranging in quality from the 6-31G(d,p) to the 6-311 +G(3df,2p). The 1,5-migration of the hydroxy H-atom in 1 yielding beta-hydroperoxyethoxy radical followed by C-C bond cleavage leading to a hydrogen-bonded [(CH2OOHCH2O)-C-.-C-...] complex (CX3) is found to be the energetically preferred pathway. At the RCCSD(T) level of theory, the 0 K energy of the rate-determining transition structure of this mechanism (TS5) is found to lie 3.6 kcal mol(-1) below the sum of the 0 K energies of the P-hydroxyethyl radical and triplet O-2. Furthermore, it is shown that CX3 can evolve into a three-component hydrogen-bonded [(CH2OOHCH2O)-O-....-C-...] complex (CX4) with a 0 K activation energy of only 0.4 kcal mol-1. Assuming that a significant fraction of the beta-hydroxyethylperoxy radicals produced in the addition of triplet O-2 to P-hydroxyethyl radical decomposes before collisional stabilization, one might expect the HO.- initiated oxidation of one molecule of ethene to yield two molecules of formaldehyde.