Abstract The hydrated calcium aluminate AFt and AFm phases are known hosts for a wide variety of chemical species. This characteristic is beneficial to those using portland cement to solidify/encapsulate radioactive waste which more often than not contains a wide variety of elements. In order to investigate the potential of the calcium aluminate hydrates as host phases for selected ions, the following experiments were carried out. Bottle hydration studies (water/solid > 1) were used to investigate the suitability of the AFt and AFm phases as hosts for iodine, one of the more mobile radioactive waste elements. Trial compositions along the AFt and AFm joins in the systems 3CaO·Al 2O 3CaSO 4CaI 2H 2O were investigated. No stable iodine end-member AFt phase was formed at room temperature. The end-member AFm phase, 3CaO·Al 2O 3·CaI 2·12H 2O, was synthesized and was designated monoiodide. Characterization was carried out using chemical analysis, scanning electron microscopy, x-ray diffraction (conventional and high temperature), thermogravimetric analysis and differential thermal analysis. Monoiodide was indexed as hexagonal and thus isostructural with previously described AFm phases. Monoiodide is stable to about 85°C. A loss of approximately four molecules of water occurs between 71° and 101°C. The lower hydrate, tentatively identified as 3CaO·Al 2O 3·CaI 2·8H 2O, is stable to approximately 300°C. The latter hydrate was observed to revert to the original hydrate on exposure to the relative humidity in the laboratory.