Deep groundwater in fractured crystalline basement has been reported from deep mines and from scientific deep wells. Highly saline brines have been described from several km depth in the continental basement of the Canadian, Fennoscandian and Ukrainian shields and elsewhere in the world. The origin of salinity is unknown and many different possibilities have been presented. We compare the compositional evolution of deep waters in the Black Forest basement, SW Germany, with those of other deep crystalline waters, and use halogen systematics (e.g. Cl/Br ratios) and other parameters of the waters to deduce the origin of their salinity. In the Black Forest the composition of deep thermal waters results from chemical interaction of surface water with the rock matrix (mainly weathering of plagioclase and mica) and from mixing of the reacted water with stagnant saline deep water. Here we show by Na/TDSβand Cl/TDSβinvestigations, by molalityβratios of the Na and Cl concentrations, and by Cl/Br systematics that these deep saline waters have a marine origin. The Cl/Br ratios in deep crystalline waters are very close to normal marine ratios (Cl/Brβ=β288 ppm basis). In contrast, Cl/Br ratios of other possible sources of salinity show distinctly different Cl/Br ratios: water derived from dissolved Tertiary halite deposits of the rift valley is in the order of Cl/Brβ=β2400 and water from dissolved Muschelkalk halite deposits has values of about Cl/Brβ=β9900. Leaching experiments on crystalline rocks, on the other hand, show that the average Cl/Br ratio of crystalline rocks is far below Cl/Brβ=β100.