A novel microcapsule adsorbent for separation of uranium from phosphoric acid solutions was developed by immobilizing the di(2-ethylhexyl) phosphoric acid--trioctyl phosphine oxide extractants in the polymeric matrix of calcium alginate. Physical characterization of the microcapsules was accomplished by scanning electron microscopy and thermogravimetric techniques. Equilibrium experiments revealed that both ion exchange and solvent extraction mechanisms were involved in the adsorption of UO22+ ions, but the latter prevailed in a wider range of acid concentration. According to the results of kinetics study, at low acidity level, the rate controlling step was slow chemical reaction of UO22+ ions with the microdroplets of extractant, whereas it changed to intraparticle diffusion at higher acid concentration. The study also attempted identification of the diffusion paths of the ions within the microcapsules, and the mechanism of change of mass transfer rate during the uptake process. The prepared microcapsules preserved their entire capacity after three cycles of adsorption, and their breakthrough behaviour was well fitted by a new formula derived from shrinking core model.