Prepn. of a component for a solid oxide fuel cell comprising a substrate coated with a polycrystalline metal oxide film of an oxide selected from (CeO2)1-x(SnO1.5)x, (ZrO2)1-x(YO1.5)x, La1-xSrxMnO3 or La1-xSrxCoyFe1-yO3, where x and y are 0-1, comprises: (a) preparing a ppte.-free starting soln. by dissolving metal cation source cpds. for each of the metal constituents of the oxide film in an aq. mixt. comprising a polymerisable organic solvent, the cations of each metal being present in the starting soln. in a molar ratio corresp. to the nominal compsn. of the oxide film; (b) heating the soln. to form a polymeric precursor comprising a polymer contg. the metal cations, the precursor being free of pptes.; (c) depositing a film of polymeric precursor onto a surface the substrate; (d) spinning the substrate to coat the substrate with a film of the polymeric precursor; and (e) calcining the deposited film in the presence of oxygen and at 600C or less to convert the film of polymeric precursor into the polycrystalline metal oxide film of average grain size of less than 0.5 m and being free of cracks or pinholes.    Also claimed is a process for the prepn. of a gas sepn. membrane for selectively transferring a component of a gaseous mixt. across the membrane, which comprises a polycrystalline metal oxide film, comprising steps (a), (b), (c) and (e) as above, where step (c) comprises depositing a film of the polymeric precursor onto the surface of a porous substrate.    Also claimed is a process for the prepn. of a gas sepn. membrane comprising a polycrystalline metal oxide film of (ZrO2)1-x(YO1.5)xor La1-xSrx-CoyFe1-yO3, where x is 0-1. The process comprises preparing a ppte.-free starting soln. by dissolving source cpds. for cations of Zr and Y, or La, Sr, Co and Fe in an aq. mixt. comprising a polymerisable organic solvent, and the carrying out steps (b), (c) and (e).    USE -   The membrane is used to separate pure oxygen from air. The fuel cell component is an electrolyte or electrode.    ADVANTAGE -   The polycrystalline metal oxide film produced in each case is free of cracks or pinholes.