Purus, Inc. is field testing a portable fiber optic chemical sensor for the semi-specific determination of ppb levels of trichloroethylene (TCE), carbon tetrachloride and chloroform in water, soil and gaseous samples. The sensor consists of a flow optrode, proprietary reagent delivery and recovery system, fiber optic transmitter-receiver, embedded micro controller, display, and communication port. The reagents react with gaseous halogenated compounds that diffuse in through a gas permeable membrane to form a colored product, and the product is detected by its absorbance of light from a 560 nm light emitting diode. The analysis time is about 3 minutes at the detection limit of a few (mu) g/L in water and may be shortened at higher concentrations and further refinements to the hardware. Field trial results are presented for a site undergoing ground water remediation for trichloroethylene. Influent water streams containing > 100 ppm TCE, effluent water streams containing < 20 ppb TCE may be routinely monitored with high precision and accuracy. These results demonstrate the viability of using this fiber optic chemical sensor as an effective low cost screening tool for assessment, monitoring, and process control applications.