Hydrological modeling of the Middle Jurassic carbonate aquifer of the Paris Basin indicated fluid residence time from 1 k.y. to 100 k.y., whereas the chemistry of waters suggested longer residence time. Here, we report independent water age estimates on the basis of atmosphere-derived noble-gas recharge paleotemperatures. The calculated paleotemperatures range from 15 to 26°C and are related to the stable isotope composition and the salinity of the waters, and the distance from the recharge. The spatial distribution of the paleotemperatures suggests (1) occurrence of old saline waters in the center of the basin which infiltrated during the warmer climate interval in the Tertiary (Eocene) and (2) dilution by recent fresh waters (early Pleistocene) flowing from the recharge. These results strongly suggest that deep aquifers in sedimentary basins can store waters for very long periods of time, in contrast to the conclusions reached from hydrological modeling.