Infrared diode laser and ultraviolet spectroscopic techniques have been used to observe the behaviour of HO2 and CH3O2 radicals generated in the gas phase by the near-ultraviolet modulated photolysis of flowing Cl2–CH4–H2–O2 and Cl2–CH4–H2O2–O2 mixtures. This has enabled measurement of the rate coefficient for the reaction HO2+ CH3O2→ products (3) which was found to have a value of (5.4 ± 1.1)× 10–12 cm3 molecule–1 s–1 at 10 Torr and 300 K. CH3O2 produced during the photolysis of Cl2–CH4–O2 mixtures displayed second-order kinetic behaviour owing to a series of reactions initiated by the reaction CH3O2+ CH3O2→ products. (1) The parameter kobs/σ(where kobs is the observed second-order rate constant) was measured for 210 ⩽λ/nm ⩽ 270 at 760 Torr and 298 K, and had a value of 1.11 × 105 cm s–1 at 250nm. σ(250 nm) was determined to be (4.25 ± 0.5)× 10–18 cm2 molecule–1, leading toi a value of (4.7 ± 0.5)× 10–13 cm3 molecule–1 s–1 for kobs. The time dependence of HDO production during the continuous near-ultraviolet photolysis of a static Cl2, CD4, CH3OH, O2 mixture has also been measured in the infrared. It seems likely that reaction (18a) accounts for the majority of the observed HDO with the remainder being formed following the production of OH radicals in the system. The observed results are consistent with the existence of two channels for reaction (18), with k18a/(k18a+k18b)≈ 0.4: CD3O2+ HO2→ HDO + CD2O + O2(18a), →CD3OOH + O2. (18b)