Evolution of stress during interfacial reactions between thin films of nickel and (001) silicon substrates has been determined at temperatures ranging from 250 to 325° C by a levered optical beam reflection technique. This technique allows accurate and in situ measurement of composite specimen curvature, thereby providing a continuous measure of stress during ongoing interfacial reactions. In addition, microstructure and phase morphology associated with these reactions have been determined by x-ray diffraction and transmission electron microscopy. Initially, Ni2Si forms and grows at the original Ni/Si interface, accompanied by an increasing net compressive stress in the composite film. Subsequently, NiSi forms and grows at the Ni2Si/Si interface, accompanied by an increasing net tensile stress in the composite film. The initial compressive stress is attributed to localized formation and growth of Ni2Si into the adjacent silicon substrate, whereas the subsequent tensile stress is attributed to formation and growth of NiSi into the adjacent Ni2Si layer. Time required to effect transition from compressive to tensile stress decreases and magnitude of both stresses decreases with increasing temperature, due to thermally activated reaction processes.