During my diploma work, titanium dioxide sol was prepared by inorganic synthesis route from TiCl4 with addition of NH4NO3 and hydroxypropyl cellulose. The prepared sol was deposited onto object glasses by dip-coating technique. After thermal treatment at 600 °C nitrogen-doped photocatalytically active thin films were prepared. These layers were then used for the photocatalytic degradation of the antibiotic oxytetracycline under UV light irradiation with a wavelength of 350 nm. The antibiotic degradation was measured spectrophotometrically every 15 minutes; the total time of photocatalytic degradation was 3 hours. The degradation of the contaminant in tap water and deionized water was monitored, based on the obtained absorption spectra of the active substance in the range between 200 and 800 nm. The influence of the pH of the solution and the initial concentration of the contaminant on the degradation rate was investigated. Calibration curves of oxytetracycline in deionized water and tap water were also constructed, from which the concentration of oxytetracycline in the solution can be determined from the measured absorbance at a wavelength of 355 nm (deionized water) and 375 nm (tap water). From the absorption spectra of these solutions before and after irradiation with UV light for three hours, the spontaneous degradation of oxytetracycline in UV light can also be followed.