Titanosilicate ETS-10 membranes were prepared by one-step seeded hydrothermal synthesis using titanium tetrabutoxide as the Ti source and fluoride ions onto alpha-alumina tubular supports. The membranes were characterized by X-ray diffraction, scanning electron rnicroscopy, temperature-programmed permeation (to measure the single gas permeances of propylene and propane), X-ray photoelectron spectroscopy and separation experiments. The duration of the hydrothermal synthesis had a strong effect both on membrane reproducibility and membrane performance measured at 30 degrees C and 100kPa in terms of propylene permeance and propylene/propane selectivity. All of the ETS-10 membranes prepared in 8-h syntheses showed propylene/propane selectivities higher than 4, with average values of selectivity and propylene permeance of 5.5 and 7.9 x 10(-8) mol/(m(2) s Pa), respectively. On the other hand, after 15 days on stream, the ETS-10 membranes lost part of their capability to separate propylene/propane mixtures. However, the propylene/propane separation could be restored by thermal treatment in air at 350 degrees C. XPS analysis indicated that the decline in separation performance was due to carbonaceous species strongly adsorbed on the ETS-10 membrane surface. (c) 2007 Elsevier B.V. All rights reserved.