To examine growth behavior of α-CoSn₃ at solid-state temperatures, kinetics of reactive diffusion between Co and Sn was experimentally observed using sandwich Sn/Co/Sn diffusion couples prepared by a diffusion bonding technique. The diffusion couples were isothermally annealed in the temperature range of 433–473 K for various times up to 744 h. Owing to annealing, an intermetallic layer consisting of CoSn₃ was formed at the original interface in the diffusion couple. The mean thickness of the intermetallic layer increases in proportion to a power function of the annealing time. The exponent of the power function takes values of 0.67 and 0.62 at 433–453 and 473 K, respectively. These values of the exponent indicate that volume diffusion predominantly controls the layer growth and interface reaction partially contributes to the rate-controlling process.