The concavitation of the diamond anvils used in a Van Valkenburg high pressure apparatus has been studied using interference techniques. The pressure distribution was then calculated from the shape of the depression. Materials with low compressional yield stress showed a maximum of pressure at the centre, whilst materials with high compressional yield stress showed a maximum near the edges. Zinc sulphide was used to illustrate this behaviour - a powdered specimen had a phase change in the central regions, a single crystal in the outer regions.