Fiber optic chemical sensors are being developed for on-line monitoring of gases and liquids. The sensors utilize novel porous polymer or glass optical fibers in which selective chemical reagents have been immobilized. These reagents react with the analyte of interest resulting in a change in the optical properties of the sensor. These sensors (or optrodes) are particularly suited to insitu monitoring of atmospheric trace contaminants and nutrient solution parameters as required in life support systems or flight experiments. Sensors have been demonstrated for detection of ammonia, ethylene, carbon monoxide, pH and humidity.