Hydrogen adsorption experiments were carried out on unirradiated and reactor-irradiated samples of gamma alumina (γ-Al 2O 2) in an effort to interpret the known enhancement by radiation of the catalytic activity of this material. The specific surface area of 85 m 2/g was not changed by irradiation, but the shapes of the hydrogen adsorption isotherms were altered. The observed monolayer capacity for hydrogen indicates that hydrogen atoms are adsorbed (at 78 °K) in one-to-one correspondence with the oxygen ions in the alumina surface. Nitrogen contamination, which is not removed by outgassing at 300 °C, has been detected. It is displaced systematically as hydrogen is adsorbed at 78 °K. Reactor irradiation appears to enhance this nitrogen release phenomenon.