Two series of samples, one of PET and another of PET, were received after chain extension at different reaction times with two new chain extenders (diimidodiepoxides). These samples showed different intrinsic viscosity and degree of branching or crosslinking. The effect of this differentiation on thermal properties was studied by thermomechanical analysis (TMA). The parameters studied were the glass transition temperature (T-g), melting temperature (T-m), and the linear expansion coefficient (alpha). It is remarkable that, in the case of PET amorphous or semicrystalline samples, two peaks appeared next to the T-g in the TMA thermogram. The first peak appeared at a temperature very close and lower to the T-g and the other peak, at higher temperature into the "cold crystallization region." The presence of two such peaks was not detected in the DSC thermogram of PET samples either in the TMS or DSC thermograms of PET. The T-g values were found to agree to within +/-1 degrees C of those obtained from DSC; on the contrary, the T-m values varied significantly from those received from DSC. The linear expansion coefficient of samples was found to increase with the degree of chain extension.