At present the water influence on the polymer crystal structure is poorly studied. Water molecules may interact with the polymer chain active sites during synthesis as well as in the processes of water adsorption from moist air. The thermoprogrammed desorption, conductivity measurement, X-ray phase analysis and high-resolution ESR techniques have been employed to study the interaction of water vapour with an emeraldine form of polyaniline (PANI). It is concluded that water influences the crystal structure of PANI and in this way changes its conductivity