Iodine oxidizes [Ru-II(NH3)(5)isn](2+) in mildly acidic (HClO4) aqueous solution at 25 OC according to the reaction I-2 + 2[RUII(NH3)(5)isn](2+) --> 2I(-) + 2[Ru-III(NH3)(5)isn](3+). The rate law is -d[Ru(II)/dt = {2k(1)[I-2] + 2k(2)[I-3(-)]}Ru(II)] with k(1) = 4.3 x 10(3) M-1 s(-1) and k(2) = 80 M-1 s(-1) at mu = 0.10 M (NaClO4). An outer-sphere electron-transfer mechanism is proposed for both terms of the rate law, with the k(1) term corresponding to the formation of I-2(-) and k(2) corresponding to the formation of I-2(-) plus I-. Subsequent reduction of Ia-by Ru(II) to form I-is expected to be fast, A value of 2.7 for log(k(22)) (the I-2/I-2(-) self-exchange rate constant) is derived from the Marcus cross relationship.