The adhesion of electrolessly deposited Ni(P) on 96 and 99.5% pure alumina was studied as a function of annealing temperature, up to 580-degrees-C. The adhesion was measured with the direct pull-off test and the peel test. The interface structure was analyzed with cross-section transmission electron micrographs. Fracture surfaces were analyzed with scanning electron microscopy/energy dispersive x-ray analysis, static-secondary ion mass spectroscopy, and x-ray photoelectron spectroscopy. The optimum annealing temperature was found to be 400-degrees-C, at which an increase in peel energy and adhesion strength by a factor of two to three was measured, with respect to the as-deposited value. The weak boundary layer, which was previously reported to be present in this system, is still present after annealing and the fracture path remains through this interfacial layer of a few nanometers in thickness. Therefore, the adhesion improvement is ascribed to stronger cohesion of the material in the weak boundary layer.