A substrate is provided with a refractory oxide coating by contacting the substrate with a dispersion of' a solid particulate phase in a liquid phase, subsequently followed by firing to produce the coating, the dispersion comprising at least a first and a second refractory oxide component. In order to produce a coating of controlled purity, e.g. for certain catalytic, electronic or other applications, the first refractory oxide component (e.g. an alumina) is obtained from a purifiable liquid or solid precursor therefor and constitutes part at least of the solid phase, and the second refractory oxide component is either obtained from a purifiable liquid or solid precursor therefor and constitutes part of the solid phase or is a purifiable liquid precursor (e.g. a water-soluble silicone) constituting part at least of the liquid phase.