This study aims to determine the sources of rare earth elements (REEs) in the geothermal and cold waters of the Ayder and tkizdere (Rize) geothermal fields (NE Turkey). The differentiation stages of REEs in water-rock interactions as well as the migration processes of aqueous complexes of rare earth elements (REEs) in the waters were evaluated. Based on results for major ions, the ikizdere geothermal water with a temperature of 61 degrees C can be classified as Na-Ca-HCO3 type water, whereas the Ayder geothermal water with a temperature of 54 degrees C can be classified as Na-Ca-CO3-SO4 type water. FREE concentrations were between 0.64-2.13 mu g/L in the ikizdere geothermal field and between 0.01-0.32 mu g/L in the Ayder geothermal field. As well as lower pH values, the waters in the tkizdere geothermal field were found to have higher REE concentrations than those of the Ayder geothermal field. Geochemical characteristics of REEs from the host rocks and water samples indicate that the REEs are derived from granite and granodiorite host rocks. Normalized REE plots for the host rocks and waters showed similar patterns, and in particular, the waters interacting with granitic rocks display a pronounced positive Eu anomaly and a less clear Ce anomaly. The positive Eu anomaly in the altered surrounding rocks is associated with argillization of feldspars in the granitic host rocks that the waters have passed through. Application of a speciation model showed that amorphous iron oxy-hydroxide complexes of REEs were significant for a pH value of nearly 9 in the Ayder geothermal waters whereas fluoride complexes are dominant in the Ikizdere geothermal waters whose pH is just below 7.