Rare earth elements and yttrium (REY), Na+, K+, Cl-, and Ca2+, were determined in water, steam, mineral scale and rock samples from the Kizildere geothermal field, Turkey. The CO2-rich parent fluid originates from a sequence of mica schists with marble intercalations. The chemistry of the parent fluid varies with location and time. The average REY composition of the fluids is derived by extrapolation to the lowest Ca concentrations. The apparent vapor-liquid partitioning factors for REY at 145 degrees C and 5 bar total absolute pressure are about 0.2, whereas for Ca2+, Na+, K+ and Cl- they are <0.05, about 0.0005, 0.0005 and about 0.02, respectively. Apparent scale-liquid distribution coefficients for REY at 145 and 190 degrees C are about 0.15 and 0.55, whereas at 100 degrees C they increase from 0.3 (La) to 1.5 (Lu). (c) 2007 Elsevier Ltd. All rights reserved.