The article presents results of the complex study of hydration and protonation processes of promising photocatalysts K₂Nd₂Ti₃O₁₀, belonging to the family of perovskite-type layered Ruddlesden–Popper phases. K₂Nd₂Ti₃O₁₀ was synthesized by solid-state method and was monitored using powder X-ray diffraction. The behavior in water has been studied by means of thermogravimetric analysis, simultaneous thermal analysis and differential scanning calorimetry. The substitution of potassium cations for protons and the water intercalation into the interlayer space of the crystal structure were observed. Proton-containing layered oxides with general formula H_xK_2−xNd₂Ti₃O₁₀·yH₂O (0.07 ≤ x ≤ 1.7, 0.06 ≤ y ≤ 0.44) have been obtained.