Chemical and isotope geothermometers, i.e. the Na-K, K-Mg, quartz and delta(18)O(SO4-H2O), have been applied to estimate the reservoir temperature of the thermal springs in the northern areas of Pakistan. The chemical types of the thermal waters and the effects of mixing of shallow cold water with the thermal end-members are discussed. These waters are neutral to slightly alkaline and have low dissolved contents. Sodium is the dominant cation in almost all the cases. In terms of anions, the hot waters of Budelas are of the SO4 type, those of Tatta Pani are of mixed character (SO4 and HCO3), and the waters from the remaining areas show HCO3 domination. An absence of tritium in Tatta Pani and Tato thermal springs indicates that they do not have any contribution of shallow young water. In the case of the Murtazabad springs, the wide range of tritium concentrations, negative correlations with surface temperature and Cl, and positive correlation between Na and Cl show that the shallow cold ground-water is mixing with thermal water in different proportions. For the mixed water of Murtazabad thermal springs, 'isochemical modelling' using the Na-K, K-Mg and quartz geothermometers indicates an equilibrium temperature in the range 185-200 degreesC. The delta(18)O(SO4-H2O) geothermometer gives relatively low temperatures for three springs, whereas two samples are close to the 185-200 degreesC temperature interval. The reservoir temperatures of Tatta Pani springs (100-120 degreesC), determined by Na-K and quartz geothermometers, are in good agreement. The delta(18)O(SO4-H2O) geothermometer gives a relatively higher range (140-150 degreesC) for most of the Tatta Pani springs. For Tato spring, the isotope and chemical geothermometers (except for the K-Mg) agree on an equilibrium temperature of about 170 degreesC. Reservoir temperatures of the remaining minor fields are not conclusive due to the lack of sufficient data. (C) 2002 CNR. Published by Elsevier Science Ltd. All rights reserved.