Interfacial reactions of Co/Si0.76Ge0.24 and Co(Si0.76Ge0.24)/Si0.76Ge0.24 by pulsed KrF laser annealing as a function of energy density and pulse number were studied. For the Co/Si0.76Ge0.24 samples annealed at an energy density of 0.2–0.6 J/cm2, three germanosilicide layers, i.e., amorphous structure and/or nanocrystal, Co(Si1−xGex), and Co(Si1−xGex)2, were successively formed along the film-depth direction. At 0.3 J/cm2 Ge segregated to the underlying Si0.76Ge0.24 film, inducing strain relaxation in the residual Si0.76Ge0.24 film. At 0.8 J/cm2 the reacted region was mostly transformed to a single layer of Co(Si1−xGex)2, whereas Ge further diffused to the Si substrate. At 1.0 J/cm2, constitutional supercooling appeared. Even the Co(Si0.76Ge0.24) film used as the starting material for laser annealing could not prevent the occurrence of constitutional supercooling at energy densities >1.6 J/cm2. The energy densities at which Co(Si1−xGex) transformation to Co(Si1−xGex)2, Ge segregation to the underlying Si, and constitutional supercooling occurred were higher for the Co(Si0.76Ge0.24)/ Si0.76Ge0.24 system than for the Co/Si0.76Ge0.24 system. Higher energy density and/or pulse number enhanced the growth of Co(Si1−xGex)2 film. In the present study, the Co/Si0.76Ge0.24 samples subjected to annealing at 0.2 J/cm2 for 20 pulses produced a smooth Co(Si0.76Ge0.24)2 film without inducing Ge segregation out of the germanosilicide and strain relaxation in the unreacted Si0.76Ge0.24 film.