Nanostructured composite materials based on polyaniline (PANI) and gold nanoparticles have been prepared by means of an osmosis based method. Several morphologies have been obtained for the pristine nanoPANI and for nanoPANI-Au composite, ranging from amorphous to sponge-like and spherical shapes. On the basis of this morphological investigation, different materials with high surface area have been selected and tested as chemical interactive materials for room temperature gas and vapor sensing. The resistive sensor devices have been exposed to different vapor organic compounds (VOCs) of interest in the fields of environmental monitoring and biomedical applications, such as toluene, acetic acid, ethanol, methanol, acetonitrile, water, ammonia and nitrogen dioxide. The effect of doping with H2SO4 has been studied for both nanoPANI and nanoPANI-Au samples. In particular, nanoPANI-Au showed sensitivity to ammonia (up to 10 ppm) higher than that to other VOCs or interfering analytes. The facile preparation method and the improved properties achieved for the polyaniline-gold composite materials are significant in the nanomaterials field and have promise for applications in ammonia vapor monitoring.