Cobalt selenide thin films have been prepared onto tin oxide glass substrates by electrodeposition potentiostatically from an aqueous acid bath containing H2SeO3 and Co(CH3COO)(2) at 50 degrees C. The electrodeposition mechanism was investigated by cyclic voltammetry. The morphological, compositional, structural, and optical properties of the deposited films have been studied using scanning electron microscopy, energy-dispersive X-ray spectroscopy, X-ray diffraction, and optical absorption techniques, respectively. The formation of cobalt selenide was confirmed to proceed via an underpotential deposition mechanism. Se-rich CoSe thin films with compact and homogeneous morphology and hexagonal crystal structure were obtained at a deposition potential of -0.5 V vs saturated calomel electrode. The electrodeposited CoSe film exhibits an optical absorption coefficient of higher than 10(5) cm(-1) and an optical bandgap of 1.53 +/- 0.01 eV. (C) 2010 The Electrochemical Society. [DOI: 10.1149/1.3468675] All rights reserved.