Co2+-doped ZnGa2O4 nanocrystals were prepared via a citrate sol-gel method. The as-obtained ZnGa2O4 nanocrystals are the only crystalline phase with an average diameter of 15-30 nm in the temperature range of 500-700 degrees C. Ga2O3 was detected when the powder was heated over 800 degrees C. So we think that the suitable fired temperature for the preparation of ZnGa2O4 nanocrystals is 500-700 degrees C. The observed absorption and emission spectra of the nanocrystalline powders in the visible and near infrared regions are characteristic of tetrahedral Co2+ ions. The crystal field parameter D-q of 402 cm(-1) and the Racah parameters B of 765 cm(-1) and C of 3420 cm(-1) were estimated for tetrahedral Co2+ ions in ZnGa2O4 nanocrystals. (c) 2006 Elsevier B.V. All rights reserved.