Single crystals of undoped and Co-doped ZnIn2Se4 were grown by the vertical Bridgman technique. The optical energy gaps of the single crystals were investigated in the temperature range of 10-300 K from the optical absorption measurements. The indirect optical energy gaps of the single crystals were found to be 1.624 eV for undoped ZnIn2Se4 and 1.277 eV for Co-doped one at 300 K. Also, the direct optical energy, gaps were given by 1.774 and 1.413 eV for undoped ZnIn2Se4 and co-doped one, respectively. The temperature dependence of the optical energy gaps was well fitted by the Varshni equation. (C) 2007 Elsevier B.V. All rights reserved.