In paleohydrogeological studies, the geochemical and isotope geochemical composition of fracture calcites can be utilised to gain information about the evolution of the composition of deep groundwaters in crystalline bedrock. The aim of our study was to investigate the latest hydrogeochemical evolution of groundwaters in the crystalline bedrock at Olkiluoto, which is the planned site for deep geological disposal of spent nuclear fuel. Samples were collected from drill cores intercepting water-conducting fractures at the upper ~500 m of the bedrock. The latest fracture calcite generations were identified using optical microscopy and electron microprobe. They occur as thin ~10-200 μm crusts or small euhedral crystals on open fracture surfaces. These latest calcite fillings were carefully sampled and analysed for the isotopic composition on carbon and oxygen. In addition, fluid inclusion homogenisation temperatures were determined on selected calcite samples. Fluid inclusion data indicated a low temperature of formation for the latest fracture calcite fillings. The δ(18)O values of calcite in these fracture fillings vary only slightly, from-7.3 to-11.5 ‰ (Vienna Pee Dee Belemnite, VPDB), whereas the δ(13)C values fluctuate widely, from-30 to+31 ‰ (VPDB). The δ(13)C values of latest calcite fillings show a systematic pattern with depth, with high and variable δ(13)C values below 50 m. The high δ(13)C values indicate active methanogenesis during the formation of the latest calcite fillings. In contrast, the present-day methanic redox environment is restricted to depths below 200-300 m. It is possible that the shift in the redox environment at Olkiluoto has occurred during infiltration of SO2-(4)-rich marine waters, the latest of such events being the infiltration of brackish waters of the Littorina Sea stage of the Baltic Sea at ~8000-3000 BP.