The structure of partially hydrated zeolite BaX containing adsorbed deuterated molecules of p- or m-xylene was studied at low temperature by neutron powder diffraction, The powder patterns were refined with the GSAS software package using the Rietveld method. The adsorption sites for water? p-xylene, and m-xylene were characterized. There are two adsorption sites for water molecules located in the sodalite cage and in the supercage, respectively. The isomers of xylene were found near the cations in the supercage in agreement with the literature data, but in addition, one "noncationic" site was found for p-xylene. The adsorption of these molecules dramatically influences the distribution of the cations. The influence of the water preadsorption on xylene adsorption selectivity is discussed.