The evolution of the phases in the cobalt-silicon reaction is studied using ultrafast optical pump-and-probe measurements. Measurements have been made on 8.5-nm thick, cobalt films sputter deposited onto Si(100) substrates, and then annealed at a series of temperatures up to 950 degreesC in helium to form Co2Si, CoSi and CoSi2. From the optical measurements, the frequency of the fundamental acoustic thickness mode of the film was determined. This frequency was used to determine the film thickness of each silicide phase, and the result is in good agreement with Rutherford back-scattering measurements. From the evolution of the damping of the oscillations, the acoustic response can be used to study the roughness of the metallic films.