Abstract <p>Ternary Ti–Sc–O thin films were prepared by atomic layer deposition (ALD) at 300°C via the alternation of reaction cycles with the TiCl₄ or Sc(MeCp)₃ precursor and H₂O as the co-reactant. Materials with [Sc]/([Ti] + [Sc]) = 13, 25, 44, 64, or 82% were prepared at various ratios between the alternating cycles. The films were characterized by spectroscopic and single-wavelength null ellipsometry, X-ray photoelectron spectroscopy (XPS), scanning electron microscopy (SEM), and X-ray diffraction. Film growth occurred within the temperature window of ALD and was substrate-inhibited. The dominant oxidation states of the metals were Ti⁴⁺ and Sc³⁺. Low scandium concentrations (up to [Sc]/([Ti] + [Sc]) = 25%) suppressed film crystallization into anatase as opposed to what was observed for TiO₂ films. The materials formed in the range [Sc]/([Ti] + [Sc]) = 44–100% had cubic structures; as the scandium concentration increases, the structure changed from disordered fluorite Sc₄Ti₃O₁₂ to Sc₂O₃-base cubic solid solution. The refractive indices <i>n</i>(<i>E</i>), absorption coefficients <i>k</i>(<i>E</i>), and optical bandgaps were well fitted to the Tauc–Lorentz model with a sharp absorption edge. They varied between the respective parameters of TiO₂ and Sc₂O₃ depending on the composition, which is topical for applied problems in optics, photonics, solar energy, and photocatalysis.</p>