Transparent glass-ceramics containing Co2+:ZnAl2O4 nanocrystals were obtained by the sol-gel process for the first time. The gels of composition 89SiO(2)-5.9Al(2)O(3)-4.9ZnO-0.2CoO (ZAS) were prepared at room temperature, and heat-treated at different temperatures. The microstructure and optical properties of the heated samples were studied by using X-ray diffraction (XRD), transmission electron microscopy (TEM), infrared spectroscopy, and optical absorption spectra. Co2+:ZnAl2O4 nanocrystals were precipitated from ZAS system and. dispersed in the SiO2-based glass during heat-treatment in the temperature range 900-1100 degreesC. Co2+ ions were located in tetrahedral sites in ZnAl2O4 nanocrystals. The Co2+:ZnAl2O4 crystallite size was in the range of 4-15 rim. (C) 2003 Elsevier Science Ltd. All rights reserved.