[en] The chlorine isotopes ³⁶Cl and ³⁷Cl have been shown to be useful tracers of groundwater, and for investigations of sources of dissolved Cl, mixing of fluids, water-rock interactions in sedimentary environments and in identifying solute sources and transport mechanisms. In addition, the radioactive isotope, ³⁶Cl, is a useful tracer for determining the residence time of groundwater. This report examines the results of Cl isotopic analysis of groundwaters from as deep as 1000 m at the Olkiluoto site in southwest Finland. Thirty-four samples were analysed for ³⁶Cl/Cl and 29 were analysed for ³⁷Cl (expressed as δ ³⁷Cl). The value δ³⁷Cl was found to stabilize at higher salinities and the maximum range of δ³⁷Cl was from about - 0.6 to +0.6 per mille. Because of this limited range and the relatively large error margins associated with the δ³⁷Cl measurement, the usefulness of this ratio appears to be limited. Therefore, the main part of this report is largely focused on ³⁶Cl. Estimation of residence time of ³⁶Cl gives results that support the presence of at least five groundwater types at Olkiluoto. The consistency of ³⁶Cl/Cl ratios in groundwaters of several widely separated, deep locations and different rock compositions, suggests that these deeper groundwaters are in secular equilibrium and, therefore, likely to be older than 1.5 million years. (orig.)