Bi₂Te₃-based thermoelectric (TE) materials have already been commercialized, of which the hygrothermal stability has a direct impact on the service reliability of TE devices, but is still confronted many challenges. This work investigated the degradation behavior of commercial n-type Bi₂Se_0.21Te_2.79 and p-type Bi_0.4Sb_1.6Te₃ TE materials during storage in 85 ℃, 85% RH hygrothermal environment for 600 h. The surfaces of n-type Bi₂Se_0.21Te_2.79 and p-type Bi_0.4Sb_1.6Te₃ TE materials were oxidized with reaction process of Bi₂Te₃+O₂→Bi₂O₃+TeO₂ and Bi₂Te₃+Sb₂Te₃+O₂→Bi₂O₃+Sb₂O₃+TeO₂, respectively. The oxidation process creates nanoscale holes and even microcracks inside the material, which leads to an overall deterioration of the electrical and thermal properties. At room temperature, the electrical conductivity of the n-type Bi₂Se_0.21Te_2.79 material drops from 9.45×10⁴ S·m^-1 to 7.79×10⁴ S·m^-1 after exposure, and ZT decreases from 0.97 to 0.79, while Seebeck coefficient of the p-type Bi_0.4Sb_1.6Te₃ material declines from 243 μV·K^-1 to 220 μV·K^-1, correspondingly, ZT decreases from 1.24 to 0.97. In conclusion, Bi₂Te₃-based TE materials have extremely poor hygrothermal stability, and their corresponding micro-TE devices need to be strictly encapsulated in service to prevent complex redox reactions between the TE materials themselves and the environmental water vapor and air.