The complex mechanism of the uncatalyzed and Ru(III)-catalyzed oxidation of toluidine blue [(7-amino-8-methylphenothiazin-3-ylidene)dimethyl ammonium chloride, TB+Cl-] (lambda (max) = 626 nm) by acidic chlorite is elucidated by a kinetic approach. Both the uncatalyzed and catalyzed reactions had a first-order dependence on the initial ClO2- and H+ concentrations ([ClO2-](0) and [H+](0), respectively). The catalyzed reaction had a first-order dependence on the initial Ru(III) concentration ([Ru(III)](0)). The overall reaction of toluidine blue and chlorite ion was as follows: TB+ + 5ClO(2)(-) + H+ = P + 2ClO(2) + 2HCOOH + 3Cl(-) + H2O, where P is (7-amino-8-methyl-5-sulfoxophenothiazin-3-ylidene)amine. Consistent with the experimental results, the pertinent reaction mechanisms are proposed.