The kinetics of the reaction CH3+ SO2(+M)→CH3SO2(+M)(1) have been studied by flash photolysis of azomethane in the presence of sulphur dioxide. The decay in the methyl radical concentration was monitored by its absorption at 216.4 nm using a novel “split-beam” photoelectric method which permits the period available for monitoring the decay to be almost trebled to 350 µS. The second order rate constant, k1, was measured to be (1.75 ± 0.25)× 108 dm3 mol–1 s–1 independent of the total pressure both in argon and nitrogen over the pressure range 6.65–26.6 kN m–2(50–200 Torr). The value is of the same order as the rate constant for the reaction of methyl radicals with oxygen and confirms the suggestion of previous workers that free radical attack on SO2 could contribute significantly to SO2 removal mechanisms and aerosol formation in polluted atmospheres.