Amorphous poly(ethylene naphthalene-2,6-dicarboxylate) (PEN) films (similar to 220 mu m thick), stored in ambient atmosphere for different periods of time and after annealing at different temperatures below T-g = 123 degrees C, for different times, have been investigated by infrared spectroscopy (FTIR), microhardness, and differential scanning calorimetry (DSC). FTIR spectroscopy and weight measurements reveal the presence of water which is easily removed by annealing of the films. Films again recover their initial weight and absorption bands after 1-2 days storage in ambient atmosphere. Samples annealed at different temperatures T-a for different times t(a) show an increasing microhardness for relatively short times of t(a). The microhardness passes through a maximum at an annealing time depending on T-a, and it decreases toward values somewhat larger than the initial ones. The changes observed in the microhardness and in the values of the excess enthalpy with storage time of the samples at room temperature depend on the physical aging as well as on the content of water of PEN films.