In this paper, we discuss some structural aspects of the proton exchange in the niobate K4Nb6O17 and also the thermal stability of the related protonic niobate phase. The exchange amount of ca. 50% of interlayer potassium ions indicates that only the potassium ions located at interlayer I was exchanged by the protons. The XRD patterns confirm the formation of an interstratificated material. The thermal stability of the protonic phase is greatly reduced compared to the potassium phase. Above approximately 315 degreesC, the protonic layers begin to dehydroxylate causing the destruction of the lamellar structure and the formation of a mixture of H-Nb2O5 and K2Nb4O11. (C) 2004 Elsevier Ltd. All rights reserved.