Zeolites of NaA and NaY types were treated with hydrochloric acid. It was found that increase of the solution acidity leads to a decationizing of the zeolites. At the same time a decrease of zeolite crystallinities was observed. For the latter, both an infrared method and X-ray diffraction were employed. The degree of crystallinity as found by the IR method is systematically higher when compared to the X-ray method, but a linear relationship exists between these values. The lowering of intensities of diffraction lines with the decrease of crystallinity is explained by changes in the electron density caused by the process of decationizing and formation of defects in the zeolite structures.