The hydrated oxygen deficient complex perovskite-related materials Sr-4(Sr2Nb2)O-11 center dot nH(2)O and Sr-4(Sr2Ta2)O-11 center dot nH(2)O were studied at high water vapour pressures over a large temperature range by electrical conductivity measurements, thermogravimetry (TG), and X-ray powder diffraction (XRPD). In humid atmospheres both materials are known to exhibit protonic conductivity below dehydration temperatures, with peak-shaped maxima at about 500 degrees C. In this work we show that the peaks expand to plateaus of high conductivity from 500 to 700 degrees C at a water vapour pressure of 1 atm. However, in situ synchrotron XRPD of Sr-4(Sr2Nb2)O-11 center dot nH(2)O as a function of temperature shows that these observations are in fact coincident with melting and dehydration of a secondary phase Sr(OH)(2). The stability of Sr-4(Sr2Nb2)O-11 center dot nH(2)O and Sr-4(Sr2Ta2)O-11 center dot nH(2)O in humid atmospheres is thus insufficient, causing decomposition into perovskites with lower Sr content and Sro/Sr(OH)(2) secondary phases. This, in turn, rationalizes the observation of peaks and plateaus in the conductivity of these materials. (C) 2009 Elsevier B.V. All rights reserved.