The kinetics and mechanism of hexacholroiridate(IV) oxidation of carboxymethyl cellulose (CMC) in aqueous solutions at a constant ionic strength of 0.1 mol dm−3 has been investigated spectrophotometrically. The experimental results showed first-order dependence in [IrCl6]2− and fractional first-order kinetics with respect to CMC concentration. An inversefractional first-order in [H+] was observed. A kinetic evidence for the formation of 1:1 intermediate complex was revealed. The reaction kinetics seems to be of considerable complexity where one chloride ion from hexacholoiridate(IV) oxidant may act as a bridging ligand between the oxidant and the substrate into the formed intermediate complex. The ionic strength was found to have a negligible influence on the reaction rates. The kinetic parameters have been evaluated, and a tentative reaction mechanism consistent with the kinetic results is discussed.