In situ time-resolved X-ray diffraction in combination with the Rietveld method was used to study the contraction/expansion behavior of a BaY faujasite zeolite during dehydration and NO2 adsorption/desorption reactions. The adsorption of water and NO2 Molecules induced a significant contraction of the zeolite unit cell. The unexpected contraction of the unit cell was closely related to the amount of water and NO, species (NO+, NO2, NO3-) inside the framework. The reaction H2O + NO2 --> NO3- + H+ had a substantial effect on the structural properties of the BaY system. The NO3- groups produced a shift in the position of the Ba cations and a reduction in the zeolite unit cell.