Hisaralan geothermal field is located at the intersection between Simav Graben and İzmir–Balıkesir Transfer Zone (IBTZ). The area is hosted by Early Miocene dacitic and rhyolitic volcanics. Clay mineralogy was investigated for the purpose of identifying the present and fossil hydrothermalsystems. HS–2 drillhole and surface clay mineralogy were examined by XRD, SEM and optical microscopy. R2 I–S and smectite are dominant clay mineral close to the surface while discerete illite and R3 I‐S are the dominant clay minerals at the deeper parts of the geothermal field. The fossil hydrothermal system developed at two stages. Initially, Fe–rich chlorite, calcite and pyrite indicative of propylitic alteration occurred. Subsequently, discrete illite and I‐S postdated the propylitic alteration. Rb/Sr isotope data from illitic minerals (illite and/or I–S) shows they are coeval and cogenetic with Early Miocene volcanism. On the other hand, active hydrothermal system is supposed to precipitate dioctahedral smectite. Activity of elements indicates the thermal waters are in equilibrium with smectite at temperatures below 100 °C. Halloysite found in the cracks and coating on the surfaces of former alteration products indicates a late phase precipitation. O and H isotopes of clay minerals indicate that low temperature geothermal systems precipitate clay minerals as smectite, and intersection of fault zones are characteristic for present day geothermal fluids Hisaralan and ıts surroundings.