The role of cerium in the suppression of gamma‐ray induced coloration in glass has been found to depend on the relative concentration of Ce3+ to Ce4+ ions as well as on the total cerium content. In a borate glass having high ultraviolet transmission, it has been found that both Ce3+ and Ce4+ ions are necessary to suppress the optical absorption bands induced in the visible region. The role of cerium can be explained on the basis of a change in its oxidation state as a result of gamma irradiation. It is postulated that the cerous ions, by the reaction Ce3+→ Ce4++e, suppress the induced visible band at 2.36 ev (525 mμ), which may result from positive hole centers. High cerous ion concentration results, however, in an induced center (Ce3++e) which absorbs in the visible at about 1.9 ev (650 mμ). The presence of Ce4+ ions near Ce3+ prevents the formation of this center possibly by the reaction Ce4++e→ Ce3+. These induced opposite changes in the oxidation state of cerium tend to maintain a balance in the ratio of Ce3+ to Ce4+ ions in the glass during irradiation, and the suppression of the visible bands depends on this ratio.