This research project investigated the interaction between natural organics acids and selected metal ions. The aim of the project was to provide quantitative data on the speciation of metal ions when placed in systems containing natural organic acids. It was envisaged that such data will assist in the risk assessment of the Drigg low level waste site in Cumbria. The formation and complexing ability of these natural organic acids is discussed and the classing of these acids into high molecular weight organic acids and low molecular weight organic acids. Initial investigations used a potentiometric technique to study the interaction between nickel and europium and selected low molecular weight organic acids which were thought to occur in significant concentrations in soils and groundwaters. These experiments confirmed existing critically assessed literature values, and provided an experimental methodology for further 'in-house' measurement of such values. In addition, studies were also performed on systems containing two competing organic acids. Studies of such systems showed no synergistic effect and that they could be modelled using individual stability constants. A comprehensive investigation was performed on the interaction of nickel and europium with humic acid. High Performance Size Exclusion Chromatography (HPSEC) was assessed for its applicability to study such systems and was found to be suitable. Stability constants were determined for europium and nickel with humic acid. All data was modelled using MINTEQA2, a geochemical speciation code. Further work on these systems was carried out by a column ion exchange technique which confirmed the HPSEC data. A comparison was then carried out between batch and column exchange with the conclusion that column gave lower metal bound to the humic at high concentrations due to competition from the resin. HPSEC was used to investigate systems of low molecular weight organic acids with humic acids. In these systems no evidence for mixed complexes was found and that there was good agreement between experimental data and model predictions. The role of humic acid at alkaline pH was also investigated. It was found that humic acid showed enhanced complexation with europium and that this was attributable to phenolic groups on the humic molecule. In conclusion, results have been generated which provide data for a number of important reactions that occur in the environment.