The first part of a series of two, this paper analyzes the degradation of pure and filled PTFE under high energy UV radiation. The polymer morphology is first investigated using differential scanning calorimetry, highlighting the respective nucleating efficiency of TiO2 and CaF2 during polymer crystallization. Then, the various polymers are exposed to excimer laser radiation and observed under an optical microscope. The results indicate that the degradation is closely connected with microstructural parameters. In pure PTFE, scattering by crystallites and reflection on piles of lamellae control the nature and extent of the degradation. In filled PTFE, nature and concentration of fillers are the most important features governing degradation. When absorbing particles are added to PTFE, the damage is restricted to the surface and photothermal processes can modify the degradation from heterogeneous to ablative, depending on the filler content.