We irradiated samples of zeolites 4A which contained different quantities of water. We measured the quantity of hydrogen released. Hydrogen radiolytic yields, present two maxima, for two water loading ratios. Hydrogen release is enhanced by the strength of the zeolite/water interaction. Hydrogen release is enhanced by the quantity of water interacting with the zeolite. a b s t r a c t Although the radiolysis of bulk water is well known, some questions remain in the case of adsorbed or confined water, especially in the case of zeolites 4A, which are used to store tritiated water. An en-hancement of the production of hydrogen is described in the literature for higher porous structures, but the phenomenon stays unexplained. We have studied the radiolysis of zeolites 4A containing different quantities of water under 137 Cs gamma radiation. We focused on the influence of the water loading ratio. The enhancement of hydrogen production compared with bulk water radiolysis has been attributed to the energy transfer from the zeolite to the water, and to the influence of the water structure organization in the zeolite. Both were observed separately, with a maximum efficiency for energy transfer at a loading ratio of about 13%, and a maximum impact of structuration of water at a loading ratio of about 4%.