Iodine-129 is a high-yield fission product of 235U and 239Pu; is produced in nuclear power plants and is therefore present in substantial quantities in radioactive wastes. In the environment, iodine exists as a range of species: iodate (IO3-), iodide (I-), elemental iodine (I2), HOI and organic species are the most common. The behaviour of iodine in the environment is linked to its speciation which can be affected by different factors such as pH, redox potential and enzymatic reduction. Previous research has shown that iodine speciation can determine its fate in the environment; however the mechanisms of redox cycling amongst the different species are not yet fully understood. This research project has focused on improving the understanding of the changes on speciation of iodine in sediment and mineral systems undergoing redox cycling reactions. The fate and changes in iodine speciation were studied under reducing and oxidising conditions, with all experiments conducted under circumneutral pH conditions. Overall the results showed that when microbial activity is promoted in a sediment system, iodate is reduced to iodide with the reduction occurring during manganese reduction. Further, when nitrate is present at high concentrations the reduction of iodate is retarded. A net release of native iodine from sediments was observed in all experiments conducted with sediments; confirming previous observations that sediment bound iodine is released from sediments under reducing conditions. Modest abiotic reduction of iodate was observed under manganese and iron reduction; and iodate reduction happened faster in a mixed system with iron and pure culture bacteria than solely by the pure culture alone or via abiotic reduction with Fe(II). When reduced experiments were exposed to air, concentrations of iodide decreased with no iodate ingrowth or losses in total iodine in solution. This suggested that iodide had been oxidised to intermediate species (I2, HOI) that were not detected, although this reaction has been described in past research. Finally, when no microbial activity is promoted, iodine remains as iodate which showed modest sorption onto sediment systems and Fe(III) oxide. Overall, these results highlight the important role that bacteria play in the reduction of iodate. This research also confirms that iodine speciation impacts on the fate of 129I throughout the environment, where it may be less mobile in an oxic environment than under reducing conditions. Moreover, some techniques of bioremediation (e.g. promoting metal reducing conditions) may cause the release of radioactive iodine to solution.