The electrical properties and water uptake of the oxygen-deficient perovskite-related compounds Sr6Ta2O11 and Sr5.92Ta2.08O11.12 have been investigated with AC conductivity, transport number measurements and thermogravimetry. In wet atmospheres, they become hydrated mainly around 500 degreesC and exhibit mixed proton and oxygen ion conductivity. Part of the water uptake toward the theoretical hydration limit takes place in a step, probably a phase transition related to oxygen vacancy disordering or proton ordering, or both. The transition is accompanied also by steps and maxima in total conductivity, believed again to be related to order-disorder reactions.