The surfaces of poly(ethylene terephthalate) obtained by irradiation with the 193-nm pulsed radiation of the ArF excimer laser (107 W/cm2) in air or the 185-nm low-intensity (2.5 mW/cm2) radiation of the mercury lamp in a vacuum were studied and compared. The XPS analysis reveals that both are depleted in oxygen, but the efficiency of the change is intensity dependent as a result of nonlinear effects at high intensity. The pulsed radiation, which gives ablative photodecomposition when the energy of the pulse is >40 mJ/cm2, yields a rough surface. SEM analysis shows that the roughness (i) has an amplitude which increases with the accumulation of pulses and reaches a maximum (1-2-μm spacing) at 10 pulses, and (ii) shows some periodicity. Since the chemical composition of the laser-treated surface does not vary with the accumulation of pulses, the increase in the advancing contact angle with water is interpreted as due to roughness. Labeling reactions and XPS have been used to probe the reactivity of the modified surfaces. Carboxylic acids, alcohols, and olefins were qualitatively and quantitatively probed. The laser treatment has a superior ability to create new functionalities. © 1986 American Chemical Society.