Electroconducting polymeric organic materials have application as sensitive components in chemical sensors. Indeed their reactions with some gases induce a variation of the electronic conductivity. We have prepared a thin film of polypyrrole by modifying a preformed Langmuir-Blodgett film by exposure to gaseous reactants and then we have tested it in organic vapours. For methanol and ethanol, the vapours caused an increase in the resistance of the film, which was rapidly reversed in air at room temperature. For acetone and ethyl acetate the response was smaller than for alcohols but however stable and reproducible. The exposure to toluene gave little resistance changes.