Composite materials made of nanocrystalline Ti: Ru: Fe: O (2: 1: 1: 2) mixed with Cr-0 or Cr2O3 were prepared by high-energy ballmilling. These materials were cold-pressed in the form of disk electrodes and used to electrolyze a chlorate electrolyte that does not contain dichromate or chromate ions. The OCl- concentration in the electrolyte was measured with respect to the electrolysis time for a variety of composite electrodes prepared by varying the milling time, t(m). In all cases, the OCl- decomposition rate, d[OCl-]/dt, was calculated from the slope of the [OCl-] vs time curve. In the case of nanocrystalline Ti: Ru: Fe: O mixed with Cr-0, d[OCl-]/dt decreases from 0.26 to 0.14 mM/ h as t(m) increases from 0 to 2 h. For t(m) > 2 h, the decomposition rate increases to 0.20 mM/ h. In the case of nanocrystalline Ti: Ru: Fe: O mixed with Cr2O3, d [OCl-]/dt is only marginally affected for t(m) < 2 h, while it reaches a value as high as 0.39 mM/ h after 5 h of milling. Through a detailed structural and surface analysis performed by using X-ray diffraction, energy-dispersive X-ray analysis, and X-ray photoelectron spectroscopy, it is shown that a chromium hydroxide layer is formed at the surface of the electrodes even if the electrolysis is conducted in a chlorate electrolyte that does not contain dichromate or chromate ions. This arises due to dissolution of Cr species in the electrolyte and their redeposition at the electrode surface under cathodic polarization. The reasons underlying the different behavior of composite electrodes made with Cr-0 instead of Cr2O3 are discussed. (c) 2005 The Electrochemical Society.