The influence of ammonia on the optical absorption of polyaniline films obtained by the electrochemical method is studied. In the presence of ammonia, the optical spectra of a polymer are found to change due to the appearance of a new absorption band with a maximum at 2.07eV and a simultaneous decrease in the intensity of bands at 1.57 and 2.91eV. The kinetics of changes showing the presence of an isobestic point at 2.71eV is elaborated. The results can be used to optimize the ammonia gas sensors for monitoring the biotechnological processes in the food industry and the environment.