Optical properties, including refractive index, photoabsorption coefficient, and film thickness, were derived with a specular extreme ultraviolet reflectivity (SEUVR) method at 13.5 nm using a newly built ultrahigh-vacuum reflectometer. Light at 13.5 nm was delivered from the 08A1BM-LSGM beamline at the National Synchrotron Radiation Research Center in Taiwan. Samples of thin-film polymethylmethacrylate (PMMA), round-robin resist (RRR), and underlayer materials were investigated. We observed an evolution in the reflectivity curve of most samples, which was ascribed mainly to a loss of thickness in the film caused by photoabsorption followed by an ablation effect. The thickness-loss rates of PMMA and RRR were (0.32±0.15) and (0.12±0.02) nm mJ -1 cm 2 , respectively, whereas the outgassing rate of RRR, scaled to a 0.4 W cm -2 power density, was estimated as 1.3×10 15 molecules cm −2 s −1 . The loss of thickness was further proven by examining the thickness profile of overexposed samples with a profilometer. From these results, we have established a satisfactory correlation between the thin-film thickness losses with a structural metric, [σ abs /doublebond equivalent per carbon atom]. The reflectivity curve stopped changing when the sample was overexposed. EUV photochemical reaction mechanisms were proposed, including outgassing by first-order exposure kinetics from the surface, which corresponded approximately to the linear ablation rate, and a slower diffusion process for outgassing from underneath the surface, which was observed for the overexposed sample as the stop of changes in the thickness.