The photodegradation of poly(methyl methacrylate) (PMMA) and vinyltriethoxysilane (VTES)-methyl methacrylate (MMA) copolymer films was studied by UV irradation (lambda = 259 nm). Evidence for the photodegradation of PMMA and copolymers was found in the different FT-IR and UV-VIS spectra. The scission of ester side-groups of PMMA were determined by using FT-IR and UV-VIS difference spectra. However, in the same process of the copolymers, both scission of the ester side-groups and crosslinking occur simultaneously. It is observed that UV irradation causes changes of the mechanical properties of PMMA and copolymers. (C) 1999 Elsevier Science Ltd. All rights reserved.