This work shows for the first time that interlayer-free cobalt oxide silica membranes can be produced via a novel silica sol-gel seeding process. The silica seed, synthesized from the Stober process, had an average particle size of 70 nm, which is incorporated into the polymeric cobalt silica sal with a particle size of less than 1 nm. Calcined cobalt oxide silica xerogels were microporous and had similar average pore sizes when the seeding ratio is less than 0.4, although the calcined silica seed itself was mesoporous. The microporous structure of the seeded cobalt oxide silica matrices was maintained by the penetration of the polymeric sol into the silica seeds at low seed ratios and effectively anchored the seeds within the polymeric sol-gel network. The best seeded membrane delivered a He permeance of 2.6 x 10(-7) mol m(-2) s(-1) Pa-1 and a He/N-2 permselectivity of 97 at 500 degrees C. whilst the perrnselectivity was only 16 for the non seeded membrane. It is proposed that the seeding technique gave rise to heterogeneous structures which can effectively block and/or repair large pores and defects of the interlayerless support compared to membranes prepared using a conventional polymeric cobalt silica sol. Ultimately this prevents infiltration of the sol during dip coating and simultaneously assists in the formation of a near defect-free membrane selective layer. (C) 2015 Elsevier B.V. All rights reserved.