The invention provides improved conditions for atmospheric pressure chemical vapor deposition (APCVD) of vanadium (IV) oxide. Specifically, higher quality vanadium oxide (particularly in the form of films) can be obtained by employing concentrations of precursors in the APCVD reaction which are substantially less than those used previously. These conditions improve the reproducibility of the films obtained by APCVD and also prevent particulate formation in the manufacturing apparatus, which in previous work had caused blockages. The films obtained have improved visual appearance, especially color, and/or have improved adhesion to a substrate. The obtained films also show a greater difference in transmission above and below the switching temperature than previous films. The invention also provides doped vanadium oxide, particularly with tungsten. Substrates (e.g. glass substrates) coated with a film of vanadium oxide are also provided. The vanadium oxide of the invention is useful for intelligent window systems, infrared modulators and data storage devices.