In this work, we performed a quantum chemical B3LYP-DFT/6-31G"* characterization of the geometries, vibrational frequencies, and relative energies of the various internal-rotation conformers of the title radicals and of the transition structures for the HOCH2CH2PO --> CH2OH + CH2O dissociation, thereby obtaining the first evidence for intramolecular hydrogen bonding in these molecules. For both the peroxy and oxy radicals, some of the equilibrium geometries were found to be stabilized by interactions between the hydroxy H and the (per)oxy O, lowering the energies by 1.5-2.5 kcal/mol, as confirmed by single point CCSD(T) calculations; it is concluded that thermalized populations at ambient temperatures should consist predominantly of the H-bonded rotamers. Furthermore, the hydrogen bond was found to persist in the transition state fur dissociation of the H-bonded HOCH2CH2O rotamers, resulting in an energy barrier calculated to be only 10 kcal/mol, in excellent accord with recent experimental results. Based on the DFT characterizations and using advanced statistical energy partitioning theories, a Master Equation analysis was performed predicting that at 298 K and 1 atm, 38% of the HOCH2CH2O radicals formed in the atmospheric HOCH2CH2OO + NO reaction dissociate "promptly" before collisional stabilization; also, the rate constant of the thermal dissociation of HOCH2CH2O was theoretically evaluated at 2.1 x 10(5) s(-1) at 298 K and 1 atm. These values, which are crucially dependent on the persistence of the H-bond during HOCH2CH2O dissociation, likewise compare favorably with recent experimental data. Isomerization of HOCH2CH2OO to OCH2CH2OOH was found to be of only minor importance to the oxidation of ethene. Theoretical results are also presented on the thermal dissociation of ethoxy and isopropoxy radicals.