The prism-coupler technique is used to investigate changes in die refractive index of PolyMethylMethAcrylate (PMMA) waveguide films exposed to low fluence KrF-laser radiation. A small (approximately 0.5%) but significant increase in index for light at 632.8 nm is observed following prolonged exposure, probably due to photochemical modification of the polymer. Weak etching of films is also evident even at a fluence as low as 6 mJ cm-2.