The Balikesir thermal waters in northwestern Turkey have discharge temperatures in the range of 31.8-98.5 degrees C, near neutral pH values of 6.40 to 8.40 and TDS contents between 327 and 2578 mg/l. Samples display variable chemical compositions changing from high-temperature Na2SO4-NaHCO3 waters to warmer CaHCO3 waters. Trace element concentrations of the waters show significant variation. Comparison between the concentrations of alkaline earth metals in waters and carbonate deposits reveals mobile behavior for Mg and Ba. The ratio of strontium to calcium concentrations in solid and liquid phases was found to be almost the same. Li/Cs and B/Cl ratios of waters and carbonates are similar suggesting the operation of a simple rock leaching process. Mineral equilibrium calculations imply that the CO2 concentration has a great effect on the chemistry of the thermal waters and that most fluid compositions are controlled by rock dissolution rather than equilibration, The chemical geothermometers applied to the Balikesir thermal waters yield a maximum reservoir temperature of 200 degrees C. The delta O-18-delta D compositions clearly indicate a meteoric origin for the waters. delta S-34 contents of sulfate in thermal waters range from -5.5 to +25.2%. Sulfur isotope compositions of some waters correspond to those of non-marine evaporates while sulfur in others is derived from sulfate reduction. The delta C-13 ratio for dissolved inorganic carbonate in the waters lies between -17.7 and +0.7%. There are also multiple sources of carbon. In high-temperature waters carbon is thought to originate from the dissolution of marine carbonates, an interpretation supported by carbon isotope compositions of marine carbonate rocks in the region. Carbon in low-temperature waters is derived from an organic source.