This work investigates the hydrostability of interlayers for inorganic membranes under harsh, yet realistic wet gas separation conditions of 550 degrees C and 75 mol % H2O. In order to avoid hydrothermal failure of traditional interlayers like gamma-A1(2)O(3), this work pursues titania as an interlayer with a strategy of either doping (5 mol %) or forming composites (25-50 mol %) with zirconium and silicon to increase the anatase-to-rutile (ATR) transition temperature and thereby improve stability. A series of powdered samples containing xTi:yZr:zSi were prepared and characterized pre-and post-hydrothermal exposure. The doped samples demonstrated an increased ATR transition temperature and no phase change even under harsh hydrothermal testing. The composite samples by contrast showed dramatically altered structural characteristics that are unsuitable for use as interlayers. Overall, the TiO2 doped with Zr achieved superior structural integrity compared to 95:5 Ti/Si or 90:5:5 Ti/Zr/Si. Substrates coated with interlayers were tested for gas (He and N-2) permeation from 100 to 500 degrees C. The He/N-2 selectivity for all membranes and conditions were around the ideal Knudsen selectivity for this gas pair. N-2 permeation pre- and posthydrothermal treatment were almost the same for all membranes, while He permeation slightly increased, particularly for the 95:5 Ti/Si membranes. These results confirm that only minor structural changes occurred after the harsh hydrothermal treatment, and that the best hydrothermal stability was provided by 95:5 Ti/Zr materials and membranes.