Co3O4 thin films were deposited onto bare and ITO (Sn-doped In2O3)-coated glass substrates directly from aqueous solutions at a temperature as low as 323 K. The principle of the deposition of the thin films was based on the slow oxidation of Co2+ ion with BrO3- ion to Co3+ ion and subsequent spontaneous hydrolysis to form a Co3O4 colloid. When the rate of oxidation was slow enough to allow heterogeneous nucleation to predominate, smooth and adherent thin films were obtained. The thin films consisted of particles less than 100 nm in diameter with low crystallinity and showed electrochromism upon applying anodic/cathodic voltage in a basic aqueous electrolyte and electrocatalytic activity for oxygen evolution. (C) 2004 Elsevier B.V. All rights reserved.