Protonation and hydration processes of layered perovskite-like oxide KCa₂Nb₃O₁₀ during the reaction with nitric acid solutions with different concentrations were studied by means of TG, STA + MS, XRD, SEM, EDX and ICP methods. It was found that despite the absence of significant changes in crystal structure, treatment of KCa₂Nb₃O₁₀ with water leads to the partial substitution of K⁺ with H⁺ (about 15% exchange), while a number of new hydrated protonated phases H_xK_1−xCa₂Nb₃O₁₀·yH₂O with higher exchange (50–90%) may be obtained using moderate acid concentrations (0.1–3 M HNO₃) varying reaction time. It was shown that production of the fully protonated form requires the minimum 3 M concentration of the acid and 24 h reaction time.