organochlorides are major contaminants in groundwater. because of this, chloroform and trichloroethylene have been the authors' primary targets in developing optical sensors. an optrode that is sensitive to trace quantities of these two contaminants was developed. the optode is constructed by inserting a stripped 200 mu m optical fiber into a 500 mu m diameter glass capillary and sealing one end with epoxy. with a microsyringe, 10 m koh solution is placed around the exposed fiber, followed by a slug of pyridine. an interface forms between the koh solution and pyridine since the two are immiscible. a mylar membrane is then used to seal the capillary, leaving an air gap between it and the pyridine. organochloride vapors are detected as they pass through the membrane, cross the air gap, and dissolve in the basic pyridine layer where a fluorescent red product is formed. the same optical fiber is used to deliver the excitation light and to collect the red fluorescence. the use of enzymes in fiber optic sensors and optical fiber sensors using antibodies are also discussed