The heat capacity of poly(trimethylene terephthalate) (PTT) has been measured using adiabatic calorimetry, standard differential scanning calorimetry (DSC), and temperature-modulated differential scanning calorimetry (TMDSC). The heat capacities of the solid and liquid states of semicrystalline PTT are reported from 5 to 570 K. The semicrystalline PTT has a glass transition temperature of 331 K. Between 340 and 480 K, PTT can show exothermic ordering depending on the prior degree of crystallization. The melting endotherm of semicrystalline samples occurs between 480 and 505 K, with a typical onset temperature of 489 K (216 degrees C). The heat of fusion of the semicrystalline samples is about 15 kJ mol(-1). For 100% crystalline PTT the heat of fusion is estimated to be 30 +/- 2 kJ mol(-1). The heat capacity of solid PTT is linked to an approximate group vibrational spectrum and the Tarasov equation is used to estimate the heat capacity contribution due to skeletal vibrations (theta(1) = 550.5 K and theta(2) = theta(3) = 51 K, N-skeletal = 19). The calculated and experimental heat capacities agree to better than +/-3% between 5 and 300 K. The experimental heat capacities of liquid PTT can be expressed by : C-p(L) (exp) = 211.6 + 0.434 T J K-1 mol(-1) and compare to +/-0.5% with estimates from the ATHAS data bank using contributions of other polymers with the same constituent groups. The glass transition temperature of the completely amorphous polymer is estimated to be 310-315 K with a Delta C-p of about 94 J K-1 mol(-1). Knowing C-p of the solid, liquid, and the transition parameters, the thermodynamic functions enthalpy, entropy, and Gibbs function were obtained. With these data one can compute for semicrystalline samples crystallinity changes with temperature, mobile amorphous fractions, and resolve the question of rigid-amorphous fractions.