Poly(trimethylene terephthalate) was investigated by isothermal thermo-gravimetry in nitrogen at six temperatures, including 304, 309, 314, 319, 324, and 336degreesC. The isothermal data have been analyzed using both a peak maximum technique and an iso-conversional procedure. Both techniques gave apparent activation energies of 201 and 192 kJ mol(-1), respectively, for the isothermal degradation of poly(trimethylene terephthalate) in nitrogen. The decomposition reaction order is calculated to be 1.0. The natural logarithms of the frequency factor based on the peak maximum and the iso-conversional techniques are 36 and 34 min(-1), respectively, for poly(trimethylene terephthalate) decomposed isothermally in nitrogen. These isothermal kinetic parameters are in good agreement with those derived by the Kissinger technique on the basis of the dynamic thermogravimetric data reported elsewhere (209 kJ mol(-1), 1.0 and 37 min(-1)). The isothermal decomposition of poly(trimethylene terephthalate) in nitrogen undergoes two processes, a relative fast degradation process in the initial period and a subsequent one with a slower weight-loss rate. The former process may be due to the removal of ester groups, trimethylene groups, and aromatic hydrogen atoms from the chain of poly(trimethylene terethphalate). The latter one may be ascribed to the further pyrolysis of the carbonaceous char.