The stress generated during thermal cycling of electron-beam-evaporated Cr-Co and Ti-Co films on quartz substrates was measured by means of the cantilever bending-beam technique between 20 and 550 °C. The thickness ratio was selected to correspond to an atom ratio of [Co]/[Cr] and [Co]/[Ti]>3. In the unreacted films, the residual stress after cycling is tensile and comparable with that in pure Co. In Co-Cr films there is only a slight increase in stress after complete reaction, whereas a marked increase in tensile stress is observed in Co-Ti films. Phase formation and reaction kinetics were investigated by Rutherford backscattering and x-ray diffraction. The dominant phases in Co-Cr are solid solutions of Cr in Co and Co in Cr with the same structure as the original films. In Co-Ti the main reaction product is the intermetallic compound Co3Ti with the Cu3Au structure. The formation of Co3Ti, which is accompanied by volume shrinkage, is diffusion controlled. The high activation energy of 2.4±0.2 eV suggests volume diffusion of Co through the growing phase as a rate limiting process.