In the present study, thermal, structural, and optical properties of sol–gel derived Zn 1−x Co x O (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 Zn 1−x Co x O synthesis undergoes three stage weight loss, in the case of lower Co concentrations (x≤0.15) before yielding Zn 1−x Co x O nanoparticles. However, for x=0.20, two stage weight loss have been observed in TG/DTA analysis. The XRD data shows single phase Zn 1−x Co x O for the lower (x≤0.15) Co concentration, while a secondary phase of Co 3 O 4 is evolved for x=0.20. The Raman and Photoluminescence data support the secondary phase formation for x=0.20 in Zn 1−x Co x O.