The thermal decomposition of engineering polyester in the atmospheres of controlled humidity has been successfully investigated by sample-controlled thermogravimetry (SCTG) and thermogravimety combined with evolved gas analysis using mass spectrometry (TG-MS). The thermal decomposition of poly(butylene terephthalate) (PBT) by conventional linear heating rate thermogravimetry (LHTG) was shown by a monotonous single decomposition step between 300 and 450 โข C. The thermal decomposition was significantly influenced by the presence of water vapor, and the amount of the residues decreased with increasing the partial pressure of water in the atmosphere. Various heating rates and mass loss rates were used for kinetic analysis in LHTG and SCTG. The apparent activation energy of the thermal decomposition of PBT was evaluated by using Friedman-Ozawa method. In nitrogen atmosphere of controlled high-humidity, the apparent activation energy estimated by SCTG was the lower than that obtained by LHTG because the influence of water vapor acts more effectively in SCTG. The TG-MS equipped with a humidity generator controlling the atmosphere revealed the presence of hydrolysis products occurred concurrently during the thermal decomposition process. A detailed thermal decomposition of PBT and the effect of water vapor are discussed.