This diploma thesis is a continuation of the final paper entitled „Preparation and characterization of CuO nanoparticles synthesized in a milireactor” in which the process of obtaining copper (II) oxide nanoparticles using milireactor was examined. The advantages of the synthesis process compared to conventional batches are faster product production at room conditions, without extreme pressures and temperatures, and a continuous production method. In the experimental part of this paper, further analyzes were performed and the optical properties of nanoparticles were examined. Nanoparticles were characterized qualitatively by the UV/VIS method and FTIR. The UV/VIS spectrum of the obtained nanoparticles shows absorption maximum below 300 nm, which is characterized by a blue shift due to the quantum confinement effect, while the FTIR spectra show high frequencies around 600 cm^-1 corresponding to vibrations of Cu-O bond. With short retention times in the millireactor (τ <6 minutes) and treatment of the sample with microwave radiation of different intensities, the obtained nanoparticles showed a uniform distribution of particle sizes corresponding to nanoparticles of 2.5 nm found in the literature. The pH of the solutions was measured and correlated with the formation of colloids of copper oxides. At the measured pH <5.66, there is no change in the color of the sample regardless of the strength of the microwave radiation.