On the basis of time-resolved spectrophotometric measurements, the formation of a novel chlorito complex, HgClO2+, was confirmed under acidic condition in the mercury-(II)-chlorite ion system. The complex formation occurs within the dead time of the stopped-flow instrument applied (tau(d) similar to 1 ms). The stability constant of HgClO2+ was determined from the initial part of the kinetic traces recorded at the characteristic 325 nm absorbance maximum of the complex: K = 41.7 +/- 1.9 M-1, epsilon(325) = 727 +/- 24 M-1 cm(-1) (I = 1.0 M (NaClO4), 25.0 degrees C). An intense formation of chlorine dioxide was observed which is due to mercury(II)-catalyzed decomposition of chlorite ion. At longer reaction times, when most of the chlorite ion is consumed, the concentration of chlorine dioxide abruptly drops to its final value. This unique feature of the reaction is attributed to an autocatalytic redox cycle which is initiated by an electron transfer step between the coordinated chlorite ion and chlorine dioxide. The proposed mechanism postulates the formation of a Cl(II) intermediate which rapidly oxidizes both ClO2- and ClO2.