The thermal waters of Kizildere and environs in the Buyuk Menderes rift zone are marked by boron concentrations of up to 32 mg/l, and flow rates of similar to250 l/s into the Buyuk Menderes River, thus increasing boron contents of the river water up to 4.4 mg/l (e.g., in 1992), for a river-water flow rate of 2 m(3)/s. These high boron concentrations poison plants, particularly citrus fruits, in irrigated agricultural areas of the rift zone. High boron contents in the thermal waters can be attributed to: (1) unstable boron-bearing mineral phases (e.g., feldspars, muscovites, tourmalines, hornblendes. and biotites) in the metamorphic rocks, proven by experimental leaching tests of various rocks; and (2) a magmatic input, corroborated by isotope analyses of delta(11)B, delta(13)C, and delta(34)S of the thermal waters. Additionally, Neogene boron deposits in northwestern Turkey have to be taken into consideration as possible sources of boron, which may contribute to these anomalous concentrations by leaching of boron-bearing minerals. Meteoric input of boron into the thermal water reservoir is ruled out because the boron contents of groundwater from the Buldan horst in the northern part of the study area is below detection limits (0.01 mg/l). There is no single reason for the high boron concentrations measured in the thermal waters of Kizildere and environs; rather, a concurrence of several natural factors is likely.