The combined chemical composition, B and Sr isotopes, and the basic geologic setting of geothermal systems from the Menderes Massif in western Turkey have been investigated to evaluate the origin of the dissolved constituents and mechanisms of water-rock interaction. Four types of thermal water are present: (1) a Na-Cl of marine origin; (2) a Na-HCO3 type with high CO2 content that is associated with metamorphic rocks of the Menderes Massif; (3) a Na-SO4 type that is also associated with metamorphic rocks of the Menderes Massif with H2S addition; and (4) a Ca-Mg-HCO3-S04 type that results from interactions with carbonate rocks at shallow depths. The Na-Cl waters are further subdivided based on Br/Cl ratios. Water from the Cumali Seferihisar and Bodrum Karaada systems are deep circulated seawater (Br/Cl = sea water) whereas water from Çanakkale-Tuzla (Br/Cl < sea water) are from dissolution of Messinian evaporites. Good correlations between different dissolved salts and temperature indicate that the chemical composition of the thermal waters from non-marine geothermal systems is controlled by: (1) temperature dependent water-rock interactions; (2) intensification of reactions due to high dissolved CO2 and possibly HCl gasses; and (3) mixing with overlying cold groundwater. All of the thermal water is enriched in B. The B isotopic composition (δ11B = 2.3‰ to 18.7‰; n = 6) can indicate either leaching of B from the rocks, or B(OH)3 degassing flux from deep sources. The large ranges in B concentrations in different rock types as well as in thermal waters from different systems suggest the water-rock mechanism. 87Sr/86Sr ratios of the thermal water are used to differentiate between solutes that have interacted with metamorphic rocks (87Sr/86Sr ratio as high as 0.719479) and carbonate rocks (low 87Sr/86Sr ratio of 0.707864).