The purpose of this paper is to understand the influence of precipitation kinetics on the U(VI) phases solubility and therefore, on the crystallinity of the phases formed. Precipitation kinetics was studied at constant pH value, by considering the influence of the method of pH control and oversaturation degree. Experiments were carried out at room temperature, anaerobic conditions and constant pH value. Characterization of solid phases were performed by scanning microscopy and X-ray diffraction. A comparison between particle size of the phase formed, solubility value obtained and precipitation method used was accomplished in order to develop a model for the U(VI) precipitation kinetics, in high concentrated saline solutions (5 M NaCl).