In this work we report thermal diffusivity and specific heat measurements in N,N-dimethylacrylamide modified poly (ethylene terephthalate) (PET) films for incorporating the dye blue Samaron HGS. These thermophysical properties were studied by changing the sample preparation conditions, such as the temperature and time of N,N-dimethylacrylamide treatment, and the time and temperature of dyeing. The results showed that the heat conduction was improved when the PET film was dyed at a temperature below the glass transition (T < 70 C). On the other hand the thermal diffusivity decreased for samples dyed at T > 70 degreesC. This behaviour can be attributed to microstructure changes in the polymer chains, as shown by the change in the amorphous/crystalline ratio, observed in the x-ray diffraction data. Pages: 2248-2254