Co has been studied extensively by many research groups as an alternative material for underbump metallization, since Co–Sn compounds show better mechanical properties than Cu–Sn compounds. Information on reactive diffusion at the solid/liquid interface is considerably important to form mechanically and electrically reliable solder joints. In the present study, the kinetics of the reactive diffusion between solid Co and liquid Sn was experimentally examined using semiinfinite Co/Sn diffusion couples prepared by an isothermal bonding technique. Isothermal annealing of the diffusion couple was conducted at temperatures in the range of 523 K to 583 K for various times up to 96 h. An intermetallic layer formed at the original Co/Sn interface in the diffusion couple during annealing. One or two intermetallic compounds among α-CoSn₃, β-CoSn₃, and CoSn₂ were identified, depending on the annealing temperature. The total thickness of the intermetallic layer was proportional to a power function of the annealing time. The overall growth rate of the intermetallic layer did not increase with increasing annealing temperature but was dependent on the kind of compound formed at the interface. The overall growth rate at 583 K was much slower than at lower annealing temperatures, since two compounds (CoSn₂ and CoSn₃) were identified at the interface, while only CoSn₃ formed at 523 K to 563 K. This indicates that the interdiffusion coefficient of CoSn₂ is much smaller than that of CoSn₃. Based on the exponent of the power function and the microstructure evolution at the moving interface, the layer growth of the compounds was controlled by volume diffusion with spheroidal growth.