C(sp(2))-H iodination of m-xylene promoted by a rhodium(III) complex supported by a redox-active ligand, ((Bu4N)-Bu-n)[(RhL3-)-L-III-(MeCN)Cl] (1(Cl)), is reported. The iodination reaction involves an electrophilic C(sp(2))-H metalation of m-xylene and successive iodine atom insertion into the rhodium(III)-carbon bond. The organometallic complex intermediate has been characterized as ((Bu4N)-Bu-n)[(RhL3-)-L-III(kappa-C-C6H4-3,5-Me-2)(H2O)] (2(Ar)) by various spectroscopic techniques as well as crystal structure analysis. The production of 3,5-dimethyliodobenzene from 2(Ar) accompanies conversion of 2(Ar) to the diiodido complex, ((Bu4N)-Bu-n)[(RhL center dot 2-)-L-III(I)(2)] (3), in which one-electron oxidized dianionic radical ligand L center dot 2- is involved. Based on these results, mechanism for the C(sp(2))-H iodination is discussed.