It has been shown earlier by the authors ( Pramana 3, 277 (1974)) that the oxidation of orthorhombic valentinite Sb 2O 3 to orthorhombic cervantite Sb 2O 4, as well as the reverse reduction, is a topotactic reaction, where the structure of valentinite is completely preserved in cervantite. Kinetic studies of the oxidation have now been done in a polycrystalline sample as well as in single crystals. The latter studies using hot-stage microscopy have enabled the rates of the reaction along different crystallographic directions to be determined. On the basis of structural and kinetic studies, a mechanism for the diffusion of oxygen atoms involving the making and breaking of bonds between (pentavalent) antimony and these oxygen atoms has been proposed. The results show that the oxidation of valentinite to cervantite is an interesting example of a solid-state reaction where the diffusion process for the oxidation is governed by structural considerations.