The Odiel river estuary shows a strong longitudinal gradient in pH and Conductivity values of the water, in agreement with the acid neutralization processes and salt-induced mixing that occur in the middle and the upper sectors of the estuarine system. The Odiel river supplies high REE dissolved (128.8 μg·l-1) that diminishing progressively until the middle zone of system (1.2 μg·l-1). In the particulate phase the REE concentration ranges between 237.8 and 48.1 mg·l-1, displaying the greater concentrations in the middle zone of the estuary. The middle and high dissolved REE are the first transferred to the particulate phase, causing a light REE depletion in the upper sectors of the mixing zone (pH between 4.2 and 5.7). The light REE transference takes place at the end of the acid neutralization processes (pH > 7), concluding in the marine zone of the estuary