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Zeolites from Ni~@ Hrabovec (Slovak Republic) were modified with solutions of NaOH. The changes of zeolites in the temperature range 20 1200 ~ 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 ~ 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.