The new perovskite oxide Sr3Ca1+xZr(1−y)−x/2Ta(1+y)−x/2O8.5−[(5x−2y)/4] belongs to the class of complex perovskites of general formula A3(B′,B″)3O9−δ. The specific composition Sr3CaZr0.5Ta1.5O8.75 has been already reported as a promising proton conducting material in a wet hydrogen atmosphere. The extent of vacancy formation and hence degree of hydration in these systems is quite extensive for a perovskite lattice. This seems related to the presence of the polarisable Ca2+ cation on the B site. In this study, we have replaced Ca with varying amounts of Zn. The extent of hydration is found to rapidly decrease with Zn substitution, as does the extent of proton conductivity. As very little water uptake is observed for compositions with more than 25% of Ca2+ replaced by Zn2+, this effect is not simply statistical. Instead, it may be supposed that extended interactions due to the polarisable Ca2+ ions facilitate the water uptake. It is proposed that mixed oxides such as Sr3CaZr0.5Ta1.5O8.75+yH2O are effectively underdoped in terms of protonic conductivity at low temperatures in wet atmospheres.