Nuclear reprocessing plants produce materials containing radioactive iodine-129. Cement can be used to immobilize and solidify radioactive iodine by fixing it in the form of the iodate ion in the mineral ettringite. Because the half-life of 129I is 15.7 million years, radioactive wastes that contain 129I require disposal by burial deep underground. In the disposal of such wastes. compacted bentonite is used as a buffer material as well as the disposal of high-level radioactive wastes. Because there is a concern that radioactive iodine could leak from disposed wastes by diffusion through compacted bentonite over a long period, the release behavior of iodine from cttringite was examined by means of eleetromigration studies in compacted bentonite. Most of the calcium and iodine is retained within the structure of iodatc-containing ettringitc (IO3-AFt), even if the chemical form of iodine changes from iodate to iodide ion as a result of reaction with ferrous ions. However, acid produced by precipitation of feme hydroxide might destroy the structure of IO3-AFt in ccmenlitious materials and cause release of iodine. It might therefore be necessary to adopt measures to prevent intrusion of acid or ferrous ions into cememittous wastes containing radioactive iodine.