fluorescence-based fiber optic sensors have been prepared which give rapid and reversible responses. a technique has been developed for the covalent chemical modification of the fiber's distal surface which is easily adapted to the smallest diameter glass optical fiber (100 mu m). the sensing layer is attached directly to the fiber surface. the layer is extremely thin and highly porous and provides high fluorescence intensity with nearly instantaneous response times. the fibers are moderately stable against bleaching and have long shelf-lives. initial efforts have concentrated on the preparation of ph-sensitive optical sensors that are useful in the ph range 4.0 to 8.0. these sensors are reversible in response to ph variation and possess signal-to-noise ratios over 250/1. the fibers are prepared using a glass surface modification followed by a polymerization step for dye immobilization. both fluorescence and absorbance-based sensors have been prepared using this technique