A novel diffusion couple preparation technique suitable for the measurement of diffusivities in molten metals and semiconductors via the long capillary method has been developed. The so-called cast coating technique involves casting of a molten alloy, which contains the impurity element to be diffused, on top of the pure solvent rods. Planar, oxide and defect free interfaces were produced at the alloy/solvent junction of the diffusion couple following basic solidification principles and a controlled atmosphere. It was noted that the thermal properties of both the molten alloy and the mould were critical in determining the location and planarity of the diffusion couple interface. The Sn/Sn-1 wt% Au and Pb/Pb-1 wt% Au diffusion couples in conjunction with moulds machined from graphite, porous graphite and a machinable ceramic were chosen as a case study.