Changes in the impedance properties of the substituted conducting polymer, poly(3,4-diphenylpyrrole) (poly(3,4-DPP)), were examined. The polymer has been shown to exhibit preferential sorption of small, chlorinated hydrocarbons from aqueous solutions. In the presence of a chlorinated hydrocarbon, the film can be electrochemically switched from an insulating to a conducting state. In the absence of the chlorinated hydrocarbon, the redox switching does not occur in aqueous electrolyte solutions. The sensing capabilities of this polymer for dichloromethane in aqueous solutions were considered. The impedance behavior of these films of poly(3,4-DPP) were found to undergo a dramatic 'on/off' response to changes in dichloromethane concentration.