AbstractThe chemical environments of europium‐exchanged NaX (Si/Al =1.16) and NaY (Si/Al = 2.29) zeolites have been investigated by means of129Xe NMR and isotherm measurements of adsorbed xenon. EuNaX and EuNaY samples with varied concentrations of Eu cations were subjected to diverse chemical and thermal treatments, namely dehydration, reduction, oxidation, and re‐reduction.Thermal analyses of hydrated EuNaX and EuNaY samples indicate that both the structural stability and the saturation concentration of water increase with increasing Eu content. For dehydrated EuNaY zeolites, the Eu3+cations tend to replace Na+ions at S2 sites and tend to be located in framework supercages; similar behavior is found for Eu2+ions after reduction. After subsequent oxidation, Eu3+ions migrate from supercages into small sodalite and/or D6R cages; similar results were deduced for samples after re‐reduction. In contrast to the behavior observed in EuNaY, Eu3+ions tend to exchange for Na+ions in the sodalite and/or D6R cages in dehydrated EuNaX zeolites, regardless of the thermal treatment; this behavior is ascribed to the existence of unlocalized S3 Na+in EuNaX sa