This work investigates the influence of hydrothermal exposure on the separation performance of sal gel derived cobalt oxide silica membranes for both single gases (He, H-2, CO2 and N-2) and binary gas mixtures (He/CO2). The surface area of the materials slightly decreased after exposed to 25 mol% water vapour at 550 degrees C for 100 h. The membranes complied with activation transport mechanism before and after hydrothermal treatment (HT), and for both single gas and gas mixture permeation. Best values were achieved for He permeance of 3.3 x 10(7) Mol m(-2) s(-1) Pa-1 at 500 degrees C and permselectivity of 479 for He/CO2. After HT, the permeance of He and H-2 decreased by 28% and 22% at 500 degrees C, respectively, while the permeance of CO2 increased and resulting in a lower He/CO2 permselectivity of 190. For gas mixtures, the He purity in the permeate side increased from 62% to 97% at 200 degrees C when the He feed molar concentration increased from 10% to 50% before HT. The He permeance remained unchanged with respect to He feed concentrations and was unaffected by the presence of CO2, although a reduction of He permeance was observed after HT exposure. The He purity in the permeate side was similar before and after HT exposure as a function of the He concentration in the feed side. Hence, the membrane matrix underwent densification though the overall pore size distribution did not broaden after hydrothermal treatment. (C) 2015 Elsevier B.V. All rights reserved.