Ba2YSnO5.5 is found to crystallize in a perovskite structure with a = 848 pm. The compound may be hydrated (Delta H degrees(hydr.) = -101.5 kJ mol(-1), Delta S degrees(hydr.) = -92.5 J mol(-1) K-1) to a high degree (maximum water content = 13.5 mol%) which leads to the formation of protonic charge carriers (OHo.) and the occurrence of very high proton conductivity with low activation enthalpy (E-a = 0.51 eV) comparable to that of BaCeO3 based materials which are the oxides with the highest proton conductivities reported so far. The conductivity contributions from native ions (V-o(..), h(.)) are found to be negligible for temperatures below 600 degrees C. In contrast to BaCeO3 based materials Ba2YSnO5.5 shows good chemical durability with respect to carbonate formation. However, structural instability becomes apparent in a high grain boundary impedance, a collapse of the structure upon reduction and the macrocracking induced by lattice strain resulting from hydration.