gamma-Radiolysis of aqueous, pH 6 solutions containing CClF2CO2- (1 x 10(-3) to 2 x 10(-2) M), HCO2- (2 x 10(-3) M), and O-2 (20-100% O-2 saturation) or N2O/O-2-(4:1 v/v)-saturated solutions of CHF2CO2- have been used as models to illustrate the high efficiency (70-100 %) of cross-termination between halogenated peroxyl radicals, here . OOCF2CO2- and O-2(.-), as opposed to the self-termination of the respective radicals. Experiments have been conducted at various [(OOCF2CO2-)-O-.]/[O-2(.-)] concentration ratios and with either of the two species in excess. The proposed mechanisms are supported by quantitative material balances. Since the final reaction products derived from CClF2CO2- are identical in nature (CO2, Cl-, F-, H+, oxalate) and yields are the same irrespective of cross- or self-termination, the conclusions were based on the H2O2 yields which are shown to differ significantly depending on the mechanism. The (OOCF2CO2-)-O-. + O-2(.-) reaction is considered to proceed via an intermediate hydroperoxide, HOOCF2CO2-, which predominantly decays via C-C cleavage into CF2O and HCO3-. Only a minor fraction (about 10%) remains as C-2-compound and ends up as oxalate. Mechanistically, the results emphasize the significance of superoxide in all systems in which peroxyl radicals are generated. With respect to halogenated hydrocarbons this is considered to be particularly relevant in, for example, the radical- and redox-induced mineralization process under aerobic conditions and in the biological metabolism of such compounds.