Two types of polyethyleneterephthalate (PET) were investigated, one nearly amorphous and the other highly crystallized. DSC analysis and mercury-in-glass dilatometry were used to determine the effect of crystalline phase content on the thermal behavior of amorphous phase. Increasing portion of crystals caused an increase in glass transition temperature (T-g) and broadening of the transition zone. Thermal expansion coefficient and specific heat decreased. The amount of rigid amorphous fraction, RAF, was calculated to be around 21-26%. Volume relaxation measurements initiated by temperature down-jump from the equilibrium above Tg to several temperatures in the vicinity of Tg showed considerably reduced relaxation rate for semicrystalline PET.