The following steps are performed for joining workpieces of metal or ceramic to each other by boundary surface diffusion at the junction location. After laying together the surfaces at which the workpieces are to be joined together, a layer is applied to the outer edge of the joint location consisting of a material compatible with the material of the parts to be joined and mechanically stable under high static pressure. The layer is fixed and sealed. Thereafter the workpieces are diffusion welded by hot isostatic pressing (HIP). A layer meeting the requirements of this process can be applied by plasma spraying onto the region of the seam at the outer edge of the junction location.