In the present study, thermal, structural, and optical properties of sol-gel derived Zn1-xCoxO (x = 0.05, 0.10, 0.15 and 0.20) nanoparticles were studied. These nanoparticles were characterized by using Thermogravimetry/Differential thermal analysis (TG/DTA), X-ray diffraction (XRD), Photoluminescence (PL), Raman, and Transmission electron microscopy (TEM). The process of sol-gel for the Zn1-xCoxO synthesis undergoes three stage weight loss, in the case of lower Co concentrations (x <= 0.15) before yielding Zn1-xCoxO nanoparticles. However, for x = 0.20, two stage weight loss have been observed in TG/DTA analysis. The XRD data shows single phase Zn1-xCoxO for the lower (x <= 0.15) Co concentration, while a secondary phase of Co3O4 is evolved for x = 0.20. The Raman and Photoluminescence data support the secondary phase formation for x = 0.20 in Zn1-xCoxO. (C) 2009 Elsevier B.V. All rights reserved.