A pulse radiolysis study of six parasubstituted benzenediazonium salts, X-C6H4N2+ [X = COOC2H5, F, H, CH3, OCH3, and N(CH3)(2)], has been carried out in strongly acidic aqueous solution (1 M HClO4), where the principal reductant is the hydrogen atom. Initially, H adducts of the diazonium salts are formed with rate constants of (2.3-9.0) x 10(8) M-1 s(-1), followed by their decay to protonated aryl radicals without the intermediacy of the diazenyl radicals that are observed when the reductant is the solvated electron. After a deprotonation reaction, the thus formed aryl radicals attack the diazonium salt mainly at the terminal nitrogen atom to afford the radical cations of the corresponding azobenzenes, (X-C6H4)(2)N-2(.+), or eventually the corresponding OH adducts, (X-C6H4)(2)N2OH., upon further reaction with water. For the dimethylamino substituent, the basicity is so high that the protonated radical cation of 4,4'-bis(dimethylamino)azobenzene is formed. Studies carried out at different pH values for this system lead to a determination of its pK(a) = 2.0. For X = COOC2H5, F, H, and N(CH3)(2), the rate-controlling step is the deprotonation reaction to afford the aryl radicals, whereas for X = CH3 and OCH3, it is the fragmentation of the H adduct to afford the protonated aryl radical. The characteristics of the corresponding OH adducts of the diazonium salts are also described briefly.