Sandıklı-Hüdai geothermal field is one of the geothermal systems in Afyon and environ, located approximately 40 km southwest of Afyon. The study area consists of volcanic, sedimentary and metamorphic rocks. Kestel greenschist formation of Paleozoic age forms the basement rock in the area while quartzite which is a member of the Kestel greenschist formation serves as the reservoir rock of Sandıklı-Hüdai geothermal system. Geothermal waters from the study area are classified as Na–SO4–HCO3 type waters. The waters plot along SO4–HCO3 end of the Cl–SO4–HCO3 triangle diagram suggesting same origin for the geothermal waters. 𝛿18O and $\delta D$ isotope ratios of the Sandıklı waters plot along the continental meteoric water line, indicating meteoric waters that were unaffected by evaporation. The tritium values imply that the fluids were deep circulating and recharged from older waters. Furthermore, oversaturation of the geothermal fluids with quartz confirms these findings showing long time residence of these groundwaters.