Polythiophene composite film was formed via in situ polymerization at room temperature onto interdigital carbon electrodes immersed into the monomer-oxidant contained mixture by adding a small amount of boron trifluoride etherate. Examinations on the gas sensitivity of polythiophene composite film to a series of vapors were carried out. It was found that the film showed good sensitivity to ammonia, as well as fast response and good reproducibility. Furthermore, the polymer film sensor could be easily recovered by high-purity N-2. (c) 2006 Elsevier B.V. All rights reserved.