Saline or brackish mineral waters have been recorded at several locations in mid-Wales and the Welsh borderlands. The main groups are centered on Llandrindod Wells and Builth Wells where maximum salinities of 5340 and 16 380 mg l−1 respectively are found. The total discharge from these springs is very low (combined 2 l s−1) yet their flow remains almost constant. Residence times of several thousand years are derived using 14C analysis, and δ18O and δ2H results suggest that the waters at Llandrindod (although not at Builth) contain a component of Late Pleistocene water. Evidence from the Br/Cl ratios and trace elements indicates that the origin of the salinity lies within the Lower Palaeozoic formations and has not been derived from evaporites. The mineral waters have undergone complete reduction of sulphate which has led to high concentrations of Ba (maximum 53.2 mg l−1) at Llangammarch Wells. The most likely origin of these waters is the deep circulation of meteoric water over a residence time of several thousand years which has risen slowly along minor fracture systems to the points of outlet. The interconnected fracture storage of these waters implied from the various lines of evidence must be of the order >30 Mm3.