We present information on the spatial variation of helium isotope ratios and several conservative geochemical constituents that serve as natural tracers to identify thermal water flow systems along the E-W trending Buyuk Menderes Graben. The air-normalized R/R-A ratios (where R = sample He-3/He-4 and R-A = air He-3/He-4) of geothermal fluids in the graben range from 3.69 to 0.2. R/R-A ratios of the high temperature fluids display regular variation along the graben, from high mantle values in the east to low crustal values in the west. The highest R/R-A ratios showing significant mantle He signatures occur at the eastern segment of the graben. In the central section, R/R-A ratios are lower and vary between 1.82 and 1.14. The lowest ratio of 0.20 R-A, indicating a value of high crustal helium up to 98%, was observed in the Germencik Field, where one of the highest temperatures recorded was 232 degrees C, at the western segment of the graben. A regional variation is also apparent in the CO2-He systematics. Mantle-derived carbon proportions were found to be generally less than 2% but similar to the R/R-A ratios, the higher mantle carbon proportions occur at the eastern segment of the graben.