Zeolites from Nižný Hrabovec (Slovak Republic) were, modified with solutions of NaOH. The changes of zeolites in the temperature
range 20–1200 °C were studied by thermal analysis (DTA, TG, ETA), X-ray analysis and REM analysis. Thermal analysis showed
that the process of dehydration started between temperatures 20 and 600 °C, over this temperature the dealumination and structural
changes have taken place. X-ray analysis and REM analysis showed the structural changes of natural zeolites and gradual loss
of cristallinity of the chemically modified zeolites.