Polyaniline (PANI) is a π-conjugated polymer that shows electric and optical property changes, when it is in contact with oxidant/reductor chemicals. In particular, the interaction between the conductive form of PANI, the emeraldine salt (ES), and the NH₃ gas results in a decrease of the polaron densities inside the band gap of the organic material. In this work, the optical-transmittance changes detected in PANI coatings as a function of the sensing ammonia gas concentrations are studied by using a nulling optical-transmittance bridge with a 632nm laser (He–Ne) source, which allows a sensitive monitoring of optical property changes. It is observed that the conductive PANI–polymethyl methacrylate (PMMA) composite coatings are sensitive for very low concentrations of HN₃ (from ∼10 to 4000ppm, depending on the PANI films’ electrical conductivity), and their optical response is reversible as the reduction gas is removed by N₂.