Nowadays, inorganic oxides have found various applications in industry and environmental protection. One of the most interesting classes of inorganic oxides is transition metal oxides. High interest is due to their interesting electrical and optical properties, as well as a suitable bandgap. As such, they have been found and applied in photocatalysis and decomposition of organic pollutants in wastewater. In this master's thesis, seven series of nanoparticles of copper(II) oxide and nickel(II) oxide were synthesized by the ultrasonic synthesis in an ultrasonic bath from copper(II) acetate monohydrate and nickel(II) acetate tetrahydrate - CuH, CuH1, CuH3, CuH5, CuE, NiH i NiE. Of the characterization methods, PXRD, FT-IR, TGA, UV/VIS, UV/VIS NIR, TEM, SEM, and XPS are used. The similarity in crystal size within the series of samples, determined by the Sherrer and Williamson - Hall method, showed that the time of ultrasonic synthesis does not affect the size of the crystallite, bandgap, and photocatalytic activity as well. The smallest optical bandgap value of 1.5 eV has the CuH sample, while the largest bandgap value of 4.025 eV has the CuE sample. The photocatalytic activity of the samples was tested on the degradation of methylene blue. It was found that with an increase in the bandgap there is a decrease in the rate constant of the photocatalytic reaction. Depending on the changes in absorbances over time, the efficiency of the photocatalyst was calculated according to which the CuH sample with the reaction rate constant of 8.1 · 10-3 min-1 has the highest efficiency and photocatalytic activity. On the other hand, the CuE sample with a reaction rate constant of 3.73 · 10-3 min-1 has the lowest efficiency and photocatalytic activity.