The Continental Intercalaire (CI) groundwaters of the Tidikelt (In-Salah region) are characterized by a high salinity and ion concentrations higher than the maximum standards for drinkability. The total dissolved solids range from 850 to 3390 mg L–1 for conductivities ranging from 1470 to 6780 μS cm–1. Their chemical facies is dominated by Cl–, SO42– and Na+, respectively. Alkali ions, Cl– and SO42– are acquired through the dissolution of Halite (NaCl) and Gypsum (CaSO4, 2H2O). CI waters have depleted δ values for 18O and 2H, corresponding to a cold end-member. This is an indication of a very homogeneous aquifer which is similar to what was observed for the CI in the eastern sub-basin (Great Oriental Erg) and for palaeowaters elsewhere in the Middle-East and Libya. Tritium analyses show that these waters are all very weakly tritiated, which is a testimony of the non-renewed character for these waters. Both δ13C and 14C measured on more than a dozen of samples also show that CI groundwaters are old, with ages comprised between 19,000 and 35,000 years with an average δ13C of –10 ‰. This means that these waters are derived from old precipitation whose features were totally different from the very scarce prevailing ones.