The ability of cement phases carrying positively charged surfaces to retard the mobility of 129I, present as iodide (I−) in groundwater, was investigated in the context of safe disposal of radioactive waste. 125I sorption experiments on ettringite, hydrotalcite, chloride-, carbonate- and sulfatecontaining AFm phases indicated that calcium−monosulfate (AFm−SO4) is the only phase that takes up trace levels of iodide. The structures of AFm phases prepared by coprecipitating iodide with other anions were investigated in order to understand this preferential uptake mechanism. X-ray diffraction (XRD) investigations showed a segregation of monoiodide (AFm−I2) and Friedel's salt (AFm−Cl2) for I−Cl mixtures, whereas interstratifications of AFm−I2 and hemicarboaluminate (AFm−OH−(CO3)0.5) were observed for the I−CO3 systems. In contrast, XRD measurements indicated the formation of a solid solution between AFm−I2 and AFm−SO4 for the I−SO4 mixtures. Extended X-ray absorption fine structure spectroscopy showed a modification of the coordination environment of iodine in I−CO3 and in I−SO4 samples compared to pure AFm−I2. This is assumed to be due to the introduction of stacking faults in I−CO3 samples on one hand and due to the presence of sulfate and associated space-filling water molecules as close neighbors in I−SO4 samples on the other hand. The formation of a solid solution between AFm−I2 and AFm−SO4, with a shortrange mixing of iodide and sulfate, implies that AFm−SO4 bears the potential to retard 129I.