[en] The zeolite is treated with silane at a temperature of 100-140₀C to silanise it in the H-form. This narrows the pores in the zeolite and the grains bind together creating cavities. The silanised zeolite and the material to be confined are brought into contact at high pressure, for example at an excess pressure of 0.5 atm. Contact is continued until the required quantity of gas or vapour has been adsorbed by the zeolite. Under the same high pressure water is added to the system which causes further narrowing of the pores. Preferably the process of silanising and treating with water is then repeated one or more times to close the pores and ensure confinement. The process is reversed by heating the product above 300₀C. The bonds formed by silanising are then broken and the confined material is liberated. This process has applications in the confinement of valuable gases such as enriched isotopes, and of dangerous gases such as radioactive waste gases. (Th.W.P.)