Thin films of diamond are of interest for technological applications such as hard coatings, heat sinks in electronic devices and miniaturized vacuum diodes(1-4). They are typically produced by chemical vapour deposition, and the presence of atomic hydrogen has been considered crucial for the growth of the diamond crystals(5-9). Some studies have claimed diamond film growth in a hydrogen-free environment(10-13), but questions remained about ( the growth conditions in those cases, Here we report the nucleation and growth of diamond by vapour deposition in a hydrogen-free, pure oxygen environment to form crystals that are heteroepitaxially aligned on a single-crystal sapphire substrate. In other words, we are able to achieve diamond growth under conditions where the oxidative ＇etching＇ of carbon must compete with its deposition. By choosing a temperature range that results in preferential oxidation of non-diamond (graphitic) carbon species to that of diamond, we are able to achieve the accumulation of diamond.