The B isotopic composition, in combination with O and H isotopes and hydrochemical tracers, is utilized to constrain the evolution of basement-hosted groundwaters via water–rock interactions and fluid infiltration from external (sedimentary) reservoirs. Two distinct groundwater types have been identified in the Central European crystalline basement (N Switzerland–SW Germany): (1) fresh groundwaters characterized by low values of δ 11B (−3.5 to −0.6‰), δ 18O (−12.0 to −10.0‰), and δD (−86.8 to −71.9‰), and (2) brackish groundwaters with distinctly heavier B, O, and H isotopic compositions ( δ 11B=+6.4 to +17.6‰, δ 18O=−9.4 to −5.6‰, δD=−67.6 to −60.8‰). Fresh groundwaters show a systematic decrease in δ 11B, related to an increase in B concentrations (and degree of total mineralization), along the pathway of groundwater migration which can only be interpreted in terms of leaching of crystalline host rocks. A δ 11B value of −3.3‰ is inferred for the crustal B source (mainly Hercynian granites) involved in the leaching process, in agreement with the known δ 11B range of granitic rocks. The evolution of brackish groundwaters, derived from crystalline basement reservoirs with little water circulation, is more complex. As indicated by B–O–H stable isotope and hydrochemical (e.g. B/Cl, Na/Cl, and Br/Cl) constraints, brackish groundwaters from the study area are influenced by admixture of sediment-derived fluids which infiltrated from Late Paleozoic (Permo-Carboniferous) and Early Mesozoic (Lower Triassic) sedimentary strata. The data presented show that B isotopes are sensitive to mixing processes of fluids derived from different crustal reservoirs and, hence, may be utilized as a tracer for constraining the internal (autochthonous) vs external (allochthonous) origin of salinity in basement-hosted groundwaters.