It is pointed out that online chemical monitoring systems can help ensure safe, environmentally sound operation of industrial processes using hazardous chemicals. Using polymer-coated surface acoustic wave sensors, the authors have demonstrated monitors that are capable of detecting dilute concentrations of volatile organic species. Using changes in both wave velocity and wave attenuation, the identity and concentration of an isolated chemical species can be determined. A polysiloxane coating has been found to provide unique properties for monitoring chlorinated hydrocarbons (CHCs) such as trichloroethylene: good discrimination of CHCs from most organic species, rapid and reversible sensor response, and low detection limits. Using this technology, a portable acoustic wave sensor (PAWS) system has been constructed. >