Abstract: The objective of this research is to examine how Cu modification can improve the photocatalytic activity of TiO2‐SiO2, to explain the correlation between the Cu concentration and the chemical state of Cu cations in the TiO2‐SiO2 matrix, and the photocatalytic activity under UV/solar irradiation. The Cu‐modified TiO2‐SiO2 photocatalysts were prepared by a low‐temperature sol–gel method from organic Cu, Si and Ti precursors with various Cu concentrations (0.05–3 mol %). The sol–gels were dried at 150 °C to obtain the photocatalysts in a powder form. The photocatalytic activity was determined by using a fluorescence‐based method of terephthalic acid decomposition. An up to three times increase in photocatalytic activity is obtained if the TiO2‐SiO2 matrix is modified with Cu in a narrow concentration range from 0.05 to 0.1 mol %. At higher Cu loadings, the photocatalytic activity of the Cu‐modified photocatalysts is lower than that of the un‐modified reference TiO2‐SiO2 photocatalyst. XRD was used to show that all Cu‐modified TiO2‐SiO2 composites with different Cu concentrations have the same crystalline structure as un‐modified TiO2‐SiO2 composites. The addition of Cu does not change the relative ratio between the anatase and brookite phases or unit cell parameters of the two TiO2 crystalline structures. We used Cu K‐edge X‐ray absorption near edge structure and extended X‐ray absorption fine structure analyses to determine the valence state and local structure of Cu cations in the Cu‐modified TiO2‐SiO2 photocatalysts. The results elucidate the mechanism responsible for the improved photocatalytic activity. In samples with a low Cu content, which exhibit the highest activity, Cu−O−Ti connections are formed, which suggests that the activity enhancement is caused by the attachment of CuII cations on the surface of the photocatalytically active TiO2 nanoparticles, so CuII cations may act as free‐electron traps, which reduce the intensity of recombination between electrons and holes at the TiO2 photocatalyst surface. At higher Cu loadings no additional Cu−O−Ti connections are formed, instead only Cu−O−Cu connections are established. This indicates the formation of amorphous or nanocrystalline copper oxide, which hinders the photocatalytic activity of TiO2. [ABSTRACT FROM AUTHOR]