Ablation is the main phenomenon which occurs on a polymer surface when a pulse of ultraviolet radiation is absorbed. The front edge of the radiation pulse is absorbed by a volume which eventually vaporizes and the rest is partially blocked by the ablation plume of products and partially transmitted to the new surface left behind after ablation. Most of this transmitted energy is transformed into heat and raises the temperature of the surface sometimes above the melting point of the polymer. Due to the transient character of the irradiation which lasts 25 ns, a rapid cooling occurs. For semicrystalline polymers like poly(ethylene terephthalate) the surface is left in an amorphous state by this cooling. In this work the amorphous depth was measured by monochromatic ellipsometry as a function of the pulse energy. It is shown that amorphization has a threshold fluence (7 mJ/cm2at 193 nm) which is lower than the ablation threshold (17 mJ/cm2). The ablation depth reaches a maximum of 850 A˚ at 193 nm when the ablation threshold is reached. Similarly the ablation depth measured at 248 nm is 1600 A˚. The amorphization depths are proportional to the radiation penetration depths in the materials.