In a method of forming a microchannel and/or microcavity structure by bonding together two elements (1, 2) having opposed plane surfaces of the same or different material, one or both surfaces having open channels and/or cavities, bonding is effected by applying to one or both element surfaces (1, 2) a thin layer (3) of a solution of a material capable of fusing with and having a lower melting point than that of the material or materials of the two element surfaces (1, 2) in a solvent which substantially does not dissolve the element surface material or materials. The solvent is then removed, and the two elements (1, 2) are brought together and heated to a temperature where the dissolved material is caused to melt but not the element surface material or materials.