The sorption rates of a-phenylglycine in sodium hydroxide solutions by Amberlite XAD-4 resin impregnated with trialkylmethylammonium chloride (Tomac), at pH 11 and 12 at 30 C, have been investigated in a batch stirred tank. By using a modified shrinking-core model that considers reversibility of the sorption process, it was shown that the amino acid sorption was controlled by particle diffusion. A more rigorous diffusion model was also used; both models led to similar diffusion coefficient values. The amino acid uptake was comparatively faster under the initial conditions of higher Tomac concentration in the resin, higher amino acid concentration in the aqueous phase and at pH = 11.