The acidity of a series of samples with different degrees of decrystallization prepared by high-energy ball milling of HNaY zeolite was studied by adsorption microcalorimetry of ammonia. Acid sites were also monitored by infrared spectroscopy using pyridine as a probe. The evaluation of the thermokinetic parameter of heat release during ammonia adsorption is a good tool to study the destruction of the crystal upon milling. Both calorimetric and infrared results suggest that the supercage structure is destroyed first during the decrystallization process of HNaY.