The joining of molybdenum to aluminium and aluminium-copper alloy using diffusion bonding has been investigated. Bond strengths have been measured by means of a simple shear jig and the joint microstructures characterized by electron microscopy and electron-probe microanalysis. Successful joints were produced by using a copper foil interlayer to form a eutectic liquid during the bonding process which helped disrupt the oxide film on aluminium and promote metal diffusion across the joint interface. When bonding commercial-purity aluminium to molybdenum, the iron present as an impurity caused a ternary eutectic liquid to form and, after solidification of the liquid phase, a thin film of Al7Cu2Fe was left behind on the aluminium. Failure of this joint occurred at a shear stress of 75 MPa, with the fracture path contained within the aluminium. With super-purity aluminium, a binary eutectic liquid was produced and the ensuing interface reaction resulted in a multi-layered structure of molybdenum-containing phases. The bond failed at the molybdenum interface at a stress of 40 MPa. When bonding aluminium-copper alloy to molybdenum without a copper interlayer, general melting at the interface via eutectic phase formation did not occur and the interface showed only localized reaction. The joint failed by separation from the molybdenum, at a stress of 25 MPa. When, however, a copper interlayer was used, fairly thick regions of multi-layered molybdenum intermetallics formed and the remaining surface was covered by a layer of Al7Cu2Mo phase. Failure of this joint occurred at a stress of 70 MPa, mainly by separation at the molybdenum interface.