This work describes an in-depth study on the electroless deposition of Co–P via the addition of a reductant to a solution on a roughened, sensitized, and activated n-PbTe thermoelectric substrate. The Co–P layer facilitates the bonding of the thermoelectric module with the solder paste and serves as a diffusion barrier that prevents severe interaction between n-PbTe and Cu or Ni electrodes. Without this layer, a eutectic reaction produces molten PbTe because of the fast diffusion of Cu; moreover, a large Cu2Te phase is formed, resulting in the depletion of Cu from the Cu electrode. In addition, Pb and Te diffuse along the grain boundaries of the Ni electrode in the Ni/n-PbTe joint. Cross-sectional images and elemental analysis results show that the Co–P layer could efficiently inhibit interdiffusion in the Ni/n-PbTe and Cu/n-PbTe joints. These results provide new insights for developing PbTe thermoelectric devices using Ni or Cu electrodes with high reliability at working temperatures.