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The Surdulica‐Bujanovac geothermal region of southeastern Serbia, with granodiorite (Surdulica) and granite (Bujanovac) aquifers (water temperatures of 125°C and 46°C, respectively) was hydrogeologically and geochemically studied using environmental isotopes from 1986 to 1990. In the Surdulica system, water, rock, and vegetation samples from 35 locations (springs, rivers, and boreholes) representing all parts of the hydrogeological system (recharge area, transition zone, and terminus with hot deep‐circulated waters) were collected for geochemical and isotope analyses (D, 18O,3H, 13C, 14C). From the recharge area to the terminus of the system, ground‐water temperatures and ionic concentrations increase two to three orders of magnitude: temperature from 3–126°C, Na+ 2.3–1127 mg/1; HCO3− 30.5–2989 mg/1; SO42− 4.8–480 mg/1; SiO2 3–90 mg/1. The hot waters, appearing at the terminus of the system (Vranjska and Bujanovac Spas locations) are the oldest, based on 14C and 3H. The δ13C attains the low negative value of −0.4‰ for hot waters, approaching those in the aquifer matrix, which are in the range between −8.4 and + 2 ‰. The δ18O and δD values in precipitation and ground water indicate that recharge takes place predominantly at the higher elevations (more than 500 m asl) and in the cold seasons. A dual‐porosity system comprised of fissures and intergranular storage controls fluid transport. The deepest flows are hydraulically isolated from fresh waters. Low 14C contents (1–6 pMC) and the absence of3 H suggest the mean residence times for the older water up to 20,000 years. A 18O shift and chemical geothermometers suggest that their base temperatures of more than 200°C are not likely.

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