Poly(ethylene terephtalate), PET, can be modified with N,N-dimethylacrylamide to obtain a better incorporation of disperse dye (Disperse Blue 79). Minimal variations in the decomposition at 10% level, melting, and glass transition temperatures, show that the thermal stability of modified PET films does not change when compared to nonmodified PET. The atomic force images show nanopeaks formation on the surface due to the modification. Modified PET films show a decrease in the contact angle and then, an increase in the superficial tension measurements, when compared to the value of 37 +/- 1 dynes.cm(-1) (nonmodified), with values liying in the range of 42-46 +/-0.5 dynes.cm(-1). The data obtained by photoacoustic spectroscopy (PAS) for dyed PET films show a dye peak at 580 nm. The data analysis of the peak area show that PET films modified with N,N-dimethylacrylamide for 15 min at 85 degrees C, dyed for 6 h at 85 degrees C with a dye concentration of 0.333 g/L, incorporate three times more dye than the nonmodified films dyed in the same conditions. By the data obtained from PAS, it was possible to calculate the depth profile of dyeing with values around 54 mu m. Factorial analyses show that the dyeing time was the most important variable. The major amount of incorporated dye was obtained by the following combination of variables: temperature and time of modifier treatment were, respectively, 72.5 degrees C and 15 min; time and temperature of dyeing were, respectively, 90 degrees C and 195 min for a dye concentration of 0.133 g/L.