Abstract Titanosilicate ETS-10 membranes were prepared by one-step seeded hydrothermal synthesis using titanium tetrabutoxide as the Ti source and fluoride ions onto α-alumina tubular supports. The membranes were characterized by X-ray diffraction, scanning electron microscopy, 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 °C and 100 kPa 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 × 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 °C. XPS analysis indicated that the decline in separation performance was due to carbonaceous species strongly adsorbed on the ETS-10 membrane surface.