Abstract: The K₂La₂Ti₃O₁₀ layered oxide was synthesized by high-temperature technique. Ion exchange properties of K₂La₂Ti₃O₁₀·H₂O were investigated by continuous potentiometric titration of its suspension in KOH with HCl. The shape of the titration curve indicates that the ion-exchange reaction of interlayer K⁺ to H⁺ proceeds at pH 7.5–8.5 at the rate as high as 5% min⁻¹. Samples obtained during the titration experiment at pH 9.8, 4.8, and 1.4 were examined by XRD, TG, ICP-AES, and SEM–EDX. Protonation already occurs at pH > 9.8, while at pH 4.8 a substitution degree of 76% is observed. K₂La₂Ti₃O₁₀ was found to be unstable against K⁺ leaching, which results in irreversible ion-exchange even when the sample is washed with distilled water. The protonation of the layered oxide significantly suppresses its photocatalytic activity in the reaction of hydrogen production from aqueous isopropanol solution, which correlates with the decreasing amount of intercalated water. Graphical abstract: [Figure not available: see fulltext.].