The Efteni and Derdin geothermal areas are located in northwestern Turkey. Relatively low-temperature springs emerge from the Duzce Fault, a normal-component-dominated fault segment of the North Anatolian Fault System. The thermal waters of the Efteni and Derdin Springs show distinct geochemical and isotopic characteristics since they originate from different geothermal reservoirs and reflect the effects of different water-rock interaction processes. Geothermometry revealed higher reservoir temperatures for the Efteni system, however a strong delta(18)O shift, interpreted as being the result of isotopic exchange at high temperatures, was observed in the Derdin system. Hydrogeological and geochemical techniques are applied to identify recharge mechanisms, water-rock interaction processes and to construct conceptual models of these geothermal systems. (C) 2008 Elsevier Ltd. All rights reserved.