Abstract <p>The photocatalytic properties of titanium oxide (TiO₂) coatings of various thicknesses synthesized by molecular layering (ML) method are studied on the surface of quartz glass plates, using photodegradation of Chicago Sky Blue 6B dye under UV-irradiation as a model reaction. The thickness and optical parameters of the coatings are determined by spectral ellipsometry, and the elemental composition is determined by scanning electron microscopy combined with energy-dispersive X-ray spectroscopy (SEM/EDS); and the morphology and continuity of the coatings are analyzed by atomic force microscopy (AFM). It is found that when the thickness of the titanium oxide layer is ≥24 nm, a single-phase, continuous titanium dioxide film is formed, completely covering the surface of the quartz substrate. It is shown that the rate of photodegradation of the dye reaches a plateau after the substrate is completely covered by the titanium oxide layer and remains virtually unchanged with a further increase in thickness. The obtained thickness–activity dependences set the minimum required thickness of TiO₂ to achieve maximum photocatalytic efficiency under the given conditions and demonstrate the effectiveness of the ellipsometry, SEM/EDS, and AFM for monitoring the structure and continuity of ML-grown coatings.</p>