Cobalt is a rare and expensive chemical element that due to its specific properties, finds numerous applications in the modern world. It is generally known as an element toxic for humans and hazardous for the environment and therefore its use is being reduced to the smallest amount possible. One of the common applications of cobalt is that it is used as a pigment component due to its extremely blue colour in some chemical compounds. Cobalt blue is one of the most important inorganic blue pigments. This master's thesis deals with the incorporation of cobalt into the willemite structure in order to prepare solid solution with colour similar to cobalt blue but with reduced threat for environment. The sol-gel synthesis of willemite with different precursors and concentrations of cobalt was carried out, and the structural, thermal and optical properties of the obtained products were examined. Willemite was prepared with two sets of precursors; first with zinc (II) acetate and cobalt (II) acetate and later with zinc (II) nitrate and cobalt (II) nitrate. Samples of willemite doped with 0, 5, 10 and 15 mol% of cobalt were prepared. The samples were annealed at temperatures from 800 to 1100 °C. X-ray diffraction analysis (XRD), Fourier transform infrared spectroscopy (FTIR), UV-Vis diffuse reflectance spectroscopy DRS, combined thermogravimetric and differential thermal analysis (TGA/DTA) were performed.