Abstract: The diffusion properties of narrow fractions of cenospheres have been investigated with respect to helium and hydrogen. It has been found that the permeability coefficients for the glass phase of the shells of cenospheres are directly proportional to the content of the glass-forming oxide SiO2 in it. The calculated values of the permeability coefficients follow the general trend with the calculated and experimental data for silicate glasses over a wide range of compositions. The experimental values of the permeability coefficients for the glass phase of the cenosphere shells at a temperature of 280°C exceed the calculated values for helium and hydrogen by a factor of 3‒24 and 12‒111, respectively. For helium at a temperature of 25°C, the experimental values of the permeability coefficients are 11‒1000 times greater than the corresponding calculated values. The selectivity for the cenospheres reaches a high level α(He/H2) = 10‒33.