Development of thermoelectric modules is very important because thermoelectric modules can recover waste heat, convert it directly into electricity, and thus enhance the energy usage efficiencies. There are many joints in a thermoelectric module which connect arrays of thermoelectric devices to the metallic conducting plates. To prevent direct contact and interactions between solders and thermoelectric materials, Ni(P) diffusion barrier layer is introduced between solders and thermoelectric substrates. The Bi2Te3-based thermoelectric materials, P-type (BixSb1-x)2Te3 and N-type Bi2(TeySe1-y)3 alloys, are the most common commercially used thermoelectric materials. In a Bi2Te3-based thermoelectric module, there are two kinds of joints, solder/Ni(P)/(BixSb1-x)2Se3 and solder(Au)/Ni(P)/Bi2(TeySe1-y)3, i.e. three kinds of interfaces, solder/Ni(P), Ni(P)/(BixSb1-x)2Te3 and Ni(P)/Bi2(TeySe1-y)3. The interfacial reactions at these contacts are examined. Although three intermediate phases, Ni3Te2, NiTe0.775 and NiTe2, are formed in the Ni/Te couples, only the NiTe2 phase is formed at the (Bi0.25Sb0.75)2Te3/Ni(P) interfaces. At the Ni(P)/Bi2(TeySe1-y)3 contact, NiTe2 and (Bi2)m(Bi2Te3)n phases are formed.