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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 HxK1−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. [ABSTRACT FROM AUTHOR]