Semiaromatic poly(ester amide)s (PEAS) were synthesized by the melt polycondensation of ethanolamine (EA) derivatives with dimethyl terephthalate and ethylene glycol in the presence of tetrabutyl titanate as a catalyst, and their crystallization and thermal properties were investigated. The introduction of an amide group into a semiaromatic polyester such as poly(ethylene terephthalate) (PET) produced PEAS (EA-modified PET polymers) with an increase in the melting point. However, these PEAS were found to decompose at a lower temperature than PET on the basis of TGA. Moreover, direct pyrolysis/mass spectrometry measurements suggested that an initial step of the thermal decomposition was a beta-CH hydrogen-transfer reaction via a six-member ring transition state at the ester-ethylene-amide unit, at which carbon-oxygen bond scission took place to yield carboxyl and N-vinylamide end groups. Furthermore, molecular orbital calculations using trimer models bis[2-[[4-(methoxycarbonyl)benzoyl]oxy]ethyl]terephthalate, N-[2-[[4-(methoxycarbonyl)benzoyl]oxy]ethyl]-4-[2-[[4-(methoxycarbonyl)b enzoyl]oxy]ethyloxycarbonyl]benzamide, and N,N'-bis[2-[[4(methoxycarbonyl)benzoyl]oxy]ethyl]terephthalamide strongly supported the idea that the beta-CH hydrogen-transfer reaction in the thermal decomposition of PEAS might occur more easily at the methylene group next to the amide group in an ester-ethylene-amide unit rather than at the methylene group next to the ester group in an ester-ethylene-ester unit. (C) 2007 Wiley Periodicals, Inc.