The crystal structure of NaY + ND3 has been determined at 20 K by powder neutron diffraction using Rietveld analysis. The cations were located at site I' (8), site II (32) and at a third site (21) in the alpha-cage. This reveals a substantial migration of the cations from the site I, which is totally emptied, and the site I' to large cages. The ND3 are in electrostatic interaction with the Na--counter-ions and in hydrogen bonding interaction with the framework oxygen atoms. This double interaction is for the first time experimentally evidenced on cationic zeolite upon ammonia adsorption. (C) 2004 Elsevier B.V. All rights reserved.