(Bi0.25Te0.75)2Te3 (p-Bi2Te3) is thermoelectric material that can harvest waste heat into useful electric power. A severe reaction between p-Bi2Te3 and Sn-based solder decreases the reliability of thermoelectric modules. Sn/p-Bi2Te3 and Sn3.0Ag0.5Cu (SAC305)/p-Bi2Te3 with and without electroless Co-P at the interfaces were investigated in this study. Without a Co-P layer, brittle SnTe, Sn3Sb2, and Bi precipitates formed at the interface. A thin layer of SnTe after reflow results in growth of a layer-type Sn3Sb2 instead of a strip-like Sn3Sb2. The addition of a Co-P layer to both systems successfully inhibited the formation of brittle intermetallic compounds. Shear test results confirmed that the Co-P diffusion barrier also effectively increased the joint strength.