The interactions between bulk rock Boom Clay and 0.1M KOH, 0.1M NaOH, 0.1M Ca(OH)2, YCW and ECW solutions, ranging in pH from 12.5 to 13.2, were examined as static batch experiments at 60°C to simulate alkaline plume perturbations, which are expected to occur in the repository due to the presence of concrete. The results indicate that the degree of geochemical perturbation in Boom Clay is dependent on the initial pH of the applied solution together with the nature of the major cation in the reactant fluids. The higher the initial pH of the media, the stronger its interaction with Boom Clay. No major non-clay mineralogical alteration of the Boom Clay was detected, but dissolution of kaolinite, smectite and illite occurred within the studied experimental conditions. The dissolution of clays is accompanied by the decrease in the layer charge, followed by a decrease in the cation exchange capacity. The highest TOC values coincide with the highest total elemental concentrations in the leachates, and correspondingly, the highest dissolution degree. However, no quantitative link could be established between the degree of organic matter decomposition and clay dissolution.