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Radioactive caesium (137Cs) and strontium (90Sr) are two of the most important contaminant radionuclides present in many terrestrial environments, as a result of accidental and approved releases from anthropogenic nuclear activity. This thesis investigates the sorption of Cs and Sr onto the surfaces of common terrestrial minerals and a mixed phase sediment. The key findings of the work were with regard to the effect of solution pH and ionic strength on changing the mechanism of Cs and Sr interaction with reactive mineral surfaces. Caesium was found to sorb to multiple sorption sites within the mixed phase sediment. The first of these was a Cs selective site at the edge of the illite interlayer, known as the frayed edge site. It was determined from electron microscopy and x-ray absorption spectroscopy that Cs was selectively incorporated and retained in the interlayer structure in inner-sphere complexes. Cs was also found to sorb to two other main sorption sites in competition with other cations. Therefore the concentrations of Cs sorbing to the mineral surfaces were greatly reduced in low pH or high ionic strength conditions. Solution pH and ionic strength also played a major role in Sr sorption. At intermediate pH Sr sorbed to the surface of illite, chlorite and goethite in outer-sphere complexes. Therefore the presence of competing ions in solution significantly reduced total Sr sorption. However, at very high pH, Sr was specifically adsorbed into inner-sphere surface complexes. This allowed significant concentrations of Sr to sorb to the mineral surface even in high ionic strength solutions. The findings of this thesis mark a significant advance in understanding the fundamental processes governing the behaviour of 137Cs and 90Sr in the environment. Particularly it shows the importance of groundwater chemistry in governing sorption behaviour.