This study examined the reaction couples of Co substrate with liquid and solid Sn-37 wt% Pb alloys, respectively. In the liquid/solid Sn-Pb/Co couples, one single-phase CoSn3 layer was found at the interface at temperatures from 210 to 250 A degrees C. The layer thickness was measured as a function of the reaction time. The layer growth rate increased with increasing temperature. The CoSn3 layer growth was linear in the early stage and then became parabolic. In addition, the similar CoSn3 growth behavior results were observed in the solid/solid Sn-Pb/Co couples from 150 to 170 A degrees C. Nevertheless, one Pb-rich phase layer accumulated at the solder/CoSn3 interface due to Sn consumption. Also, the reaction kinetics of Sn-37 wt% Pb/Co was symmetrically studied at various temperatures. The chemical reaction constants, diffusion constants, and the relevant activation energies were reported.