Using pulse radiolysis and steady-state gamma-radiolysis techniques, it has been established that, in air-saturated aqueous solutions, peroxyl radicals CH(2)HalOO(center dot) (Hal = halogen) derived from CH2O2 and CH2Br2 react with dimethyl selenide (Me2Se), with k on the order of 7 x 10(7) M-1 s(-1), to form HCO2H, CH2O, CO2, and CO as final products. An overall two-electron oxidation process leads directly to dimethyl selenoxide (Me2SeO), along with oxyl radical CH(2)HalO(center dot). The latter subsequently oxidizes another MeSe molecule by a much faster one-electron transfer mechanism, leading to the formation of equal yields of CH2O and the dimer radical cation (Me2Se)(2)(center dot+). In absolute terms, these yields amount to 18% and 28% of the CH2ClO center dot and CH2BrO center dot yields, respectively, at 1 mM Me2Se. In competition, CH(2)HalO(center dot) rearranges into (CH)-C-center dot(OH)Hal. These C-centered radicals react further via two pathways: (a) Addition of an oxygen molecule leads to the corresponding peroxyl radicals, that is, species prone to decomposition into H+/O-2(center dot-) and formylhalide, HC(O)Hal, which further degrades mostly to H+/Hal(-) and CO. (b) Elimination of HHal yields the formyl radical H-C-center dot = O with a rate constant of about 6 x 10(5) S-1 for Hal = Cl. In an air-saturated solution, the predominant reaction pathway of the H-C-center dot =-O radical is addition of oxygen. The formylperoxyl radical HC(O)OO center dot thus formed reacts with Me,Se via an overall two-electron transfer mechanism, giving additional Me2SeO and formyloxyl radicals HC(O)O-center dot. The latter rearrange via a 1,2 H-atom shift into C-center dot(O)OH, which reacts with O-2 to give CO2 and O-2(center dot-). The minor fraction of H-C-center dot = O underooes hydration, with an estimated rate constant of k approximate to 2 x 10(5) S-1. The resulting HC center dot(OH)(2) radical, upon reaction with O-2, yields HCO2H and H+/O-2(center dot-). Some of the conclusions about the reactions of halogenated alkoxyl radicals are supported by quantum chemical calculations [B3LYP/6-31G(d,p)] taking into account the influence of water as a dielectric continuum [by the self-consistent reaction field polarized continuum model (SCRF=PCM) technique]. Based on detailed product studies, mechanisms are proposed for the free-radical degradation of CH2Cl2 and CH2Br2 in the presence of oxygen and an electron donor (namely, Me2Se in this study), and properties of the reactive intermediates are discussed.