A technique of reactive d.c. magnetron sputtering with r.f. substrate bias has been evolved to give metal oxide films which exhibit heat reflecting properties while remaining highly transparent. Films of indiumtin, indium and cadmiumtin oxide have been deposited onto plastic sheet at room temperature at rates of greater than 0.5μm min.-1 Preliminary assessments of durability with accelerated weathering with exposure to high U. V. levels and high humidities have Given very encouraging results. The properties achieved with a sinale coating of about 300 nm of oxide to a 50μm thick P.E.T. sheet are visible transmittances of over 70% with heat emissivities lower than 0.3. These properties are commensurate with them providina an energy and cost effective addition to new and existing windows.