Experimental results on the gas-sensing properties of polymer thin films are presented. Langmuir-Blodgett films were formed from a 2:1 M mixture of poly[3-(butylthio)thiophene] (PBTT) and arachidic acid. A stable monolayer was prepared at 20 degrees C on the water surface and it was reiteratively transferred onto various substrates by vertical dipping. The visible spectra of the films indicate a HOMO-LUMO separation of around 1.8 eV with an absorption maximum at about 520 nm. Electrical conductivity measurements performed in controlled atmosphere showed large gas sensitivity to electron-acceptor gases such as nitrogen dioxide at a working temperature of the active layer of about 50 degrees C.