In studies directed towards 1) the design of disperse dyes for polyester (PET) fibers for applications requiring high lightfastness and 2) determining the contribution of PET substrates to the fading of various disperse dye types, microscopic analyses were used to characterize the properties of dyed and undyed PET films following natural and artificial light exposures. In this investigation, high-grade PET films were exposed to Arizona and Florida sunlight and to the artificial light of a standard weatherometer. The results indicated that artificial light exposures caused more dye and polymer degradation than natural light and that a significant level of dye fading could be attributed to the presence of products arising from PET degradation.