Polypyrrole (PPy) thin films were prepared electrochemically at a constant potential. Gas-sensing behaviors, including reproducibility, sensitivity, and response time to various benzene, toluene, ethylbenzene, and xylene (BTEX) compound concentrations, were investigated. BTEX compounds were found to be able to compensate for the doping level of PPy and, hence, decrease the conductivity of PPy on exposure to them. A reasonable reproducibility of the resistance change (DeltaR) was obtained. The sensitivity for each compound was 2.3 m Ohm /ppm (benzene), 0.4 m Ohm /ppm (toluene), 8.3 m Ohm /ppm. (ethylbenzene), and 2.9 m Ohm /ppm (xylene). An adsorption model correlated well with the experimental results and was used to interpret the sensing behaviors. The parameters of this model, including the adsorption equilibrium constant and the AR caused by a pseudomonolayer of the detecting layer {[m(r(1) -(0))]/n, where rn is the number of active sites on the pseudomonolayer; r(1) and r(0) are the site resistances when the site is vacant and occupied, respectively; and n is the thickness of the film}, were determined. According to the parameters, toluene vapor had the most prominent effect in undoping PPy film but the poorest affinity to the active sites of the film. On the other hand, ethylbenzene showed the highest affinity to PPy film compared to the other BTEX compounds and consequently led to the highest sensitivity for such a sensor.