The kinetics and mechanism of hexachloroiridiate (IV) oxidation of folic acid (vitamin B-9) (FA) in aqueous perchlorate solutions at a constant ionic strength of 2.0 mol dm^-3 were investigated spectrophotometrically. The experimental results showed first-order dependence on [IrCl₆]^2- and fractional-first-order kinetics with respect to the folic acid concentration. A kinetic evidence for the formation of 1:1 intermediate complex was revealed. The influence of hydrogen ion concentration on the reaction process indicated the increase in the oxidation rates with decrease in the hydrogen ion concentration, i.e. the reaction is acid-inhibition. The activation parameters were evaluated and a tentative reaction mechanism consistent with the kinetic results is suggested.