In this study, behavior and migration processes of rare earth elements (REEs) were determined in the thermal and cold waters of the geothermal fields in the Eastern Black Sea Region. The temperatures of the geothermal waters are between the range of 38 and 60 degrees C. The water types are Na-Ca-SO4 for Sarmasik (Ordu) field, Na-Ca-HCO3 for Ikizdere (Rize) field, Na-Ca-CO3-SO4, for Ayder (Rize) field and Na-HCO3-Cl for Savsat (Artvin) field. All geothermal fields are in alkaline geothermal character (pH: 7-9.3) and have very low REE + Y concentrations, slightly significant Ce contens and significantly positive Eu anomalies (except for Ayder geothermal field). REE+Y content, fractionation and positive Eu anomalies of the investigated water are controlled by water/rock ratio, pH and minerals in the host rocks. According to the thermodynamic calculations, it was determined that the mobility of the REEs in geothermal fields were mainly controlled by Ln(3+), LnOH(2+), LnCl(2+), Ln(SO4)(2)(-), LnSO(4)(+), LnHCO(3)(2+), Ln(CO3)(2)(-) and LnCO(3)(+) complexes.