It was discovered that the noble gases Ar, Kr and Xe can be trapped firmly in type A zeolites as well as in mordenites and chabazites when the molecular sieves are hydrothermally vitrified at temperatures between 340 and 650°C in the presence of pressurized gas. The occurrence of trapping in an amorphous nonzeolitic species was demonstrated by X‐ray diffraction, neutron scattering and electron scattering. The change in structure of loaded zeolites is also manifested by a substantial loss in sorption capacity for water as well as an almost complete disappearance of the specific surface area. – The gas release rates at temperatures below 550°C were found to be extremely low. The activation energies for the diffusion of Kr through amorphous Ca and Mg exchanged 5 A zeolite were determined to be 209 ± 15 and 187 ± 16 kJ/mol respectively. The estimated diffusion coefficients are exceedingly small. The trapped gas is neither liberated by hot water nor by liq. Rb at 150°C.