A 5.28 m-long ice core was extracted from a major cave ice body in the Mammuthöhle cave system. The upper ~1.2 m of ice most likely originate from precipitation fallen before the 1960s (based on <8.5 TU). Characteristic fluctuations in electrical conductivity were observed in the cave ice profile, which seem to mirror the fluctuation of karst and surface water in the water supply of the ice accumulation. The stable isotope composition does not support the hypothesis that ice layers with low conductivity are formed by freezing out of water vapour. Isotope fractionation effects during the freezing process are indicated by the enrichment of heavy stable isotopes (²H, ¹⁸O) in the ice compared to the potential sources (local precipitation, karst water) and by the characteristically low <i>d</i>-excess values. In addition, the cave ice water line shows a slope coefficient of 8.13. A two-component open-system model (i.e. a depleted component mixed with the freezing water) can adequately explain the measured isotopic compositions of the Saarhalle cave ice.