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Pulse radiolysis of air-free aqueous methyl viologen (MV2+) solutions was carried out at various pH. The attack of eiq on MV2+, with k(eiq+MV2+) = 7.5 x 1Olo dm3 mo1-l s-l, leads to the formation of the long-lived radical cation (MV' +), which possesses two absorption maxima at 392.5 nm (E,,,,, = 4200 m2 mol-') and 600 nm ( E ~, , = 1450 m2 mol-l). The H-atoms react with MV2+ at pH 1 forming two species, which have superimposed absorption bands. By means of a computer simulation they are resolved in the absorptions belonging to: (1) a protonated form of the radical cation (MV'+H+), which is produced with k(H + MV2+) = (3.5 f 0.2) x lo8 dm3 mol-l s-l, has 2 absorption maxima at 390 nm (E,,, = 1700 m2 mol-I) and 595 nm ( E ~, : , = 760 m2 mol-l) and decays by second-order kinetics with k = 3.5 x lo9 dm3 mol-' s-l; (2) an H-adduct (MVe2+H) on the ring carbon, which is formed with 2467 80-2

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