The thermal endurance of poly(butylene terephthalate) was evaluated by both consolidated long-term conventional life tests and a short-term analytical procedure. The latter is based on oxidative stability measurements by isothermal differential calorimetry in a pure oxygen flow. The capability of the analytical technique to provide an activation energy for the thermo-oxidation process was verified. The results obtained by the two methods were compared and discussed. It is shown that the temperature index derived by the analytical procedure can be close to that obtained by the conventional tests, provided that a suitable selection of the test temperatures and failure criteria is made; however, benefits in term of test time shortening are lower than those previously obtained for polyolefin-based materials.