The interfacial reactions between various molten metals and solid plates were investigated in this diffusion couple study. The molten metals were pure magnesium, pure aluminium, aluminium-rich Al-Mg alloy, and aluminium-rich Al-Cu alloys, and the solid plates were pure nickel plate, alumina plate, and nickel-plated alumina plate. The interfacial reactions in the diffusion couples were determined by using optical microscopy, scanning electron microscopy and electron probe microanalysis in regard to the formation of intermetallic phases, the dissolution rates of the nickel plates, and the morphology of the interfaces. Mg2Ni phase was found in the pure Mg/Ni plate diffusion couples, and the Al3Ni and Al3Ni2 phases were observed in the pure Al/Ni plate and Al-alloys/Ni plate diffusion couples. In the Al-Cu alloy/Ni-plated alumina plate diffusion couple, Al2O3 formed at the interface, while spinel particles were found in the diffusion couples of Al-7.4 wt % Mg alloy/Ni-plated alumina plate. Experimental difficulty was encountered in preparing the diffusion couples with alumina plate, and a gap existing at the interface prohibited reactions between the molten metal with alumina plate.