The oxidation of hydroxylamine by [IrCl6](2-) has been studied spectrophotometrically in deoxygenated aqueous solutions in the range of pH 4-9 at 25 degrees C. The reaction is catalyzed by Cu2+, Fe2+, and impurities of aquochloroiridium complexes. Oxalate is a very effective inhibitor of catalysis by copper and iron ions. With excess hydroxylamine, the reaction follows pseudo-first-order kinetics, and the stoichiometric ratio (Delta n(Ir(IV))/Delta n(hydroxylamine)) is 1.05 at pH 5.9. Over the pH range 4.2-8.8, the empirical rate law is -d[IrCl62-]/dt = k[IrCl62-][NH2OH](tot), with k = k(1)K(a1)/([H+] + K-a1) + k'K-a1/([H+]([H+] + K-a1)), where K-a1 is the dissociation constant of NH3OH+. Least-squares fitting yields k(1) = (17.05 +/-0.47) M-1 s(-1) and k' = (2.59 +/- 0.09) x 10(-6) s(-1) at ionic strength of 0.1 M (adjusted by NaClO4) and 25 degrees C. The kinetic isotope effects (KIE) (k(H)/k(D)) for k(1) and k' are 4.4 and 9.8, correspondingly. A mechanism is inferred in which k(1) corresponds to concerted proton-coupled electron transfer (PCET) and k' corresponds to electron transfer from NH2O-. In this mechanism, the large KIE for k' is due almost entirely to the equilibrium isotope effect for the pK(a) of NH2OH.