The kinetic behavior of the extraction of aluminum and beryllium with dinonylnaphthalene sulfonic acid (DNNSA), bis(2-ethylhexyl)phosphoric acid (D2EHPA) and their mixture was studied using a stirred transfer cell. The extraction rate obtained with DNNSA is 5-50 times larger than that obtained with D2EHPA, The rate-determining step in the extraction with D2EHPA is the reaction between metal ion and dimeric D2EHPA. The diffusion steps for the DNNSA reverse micelles in the organic phase and for the metal ions in the aqueous phase, however, govern the overall rate of extraction with DNNSA, The extraction rate obtained with the DNNSA-D2EHPA mixed micelles is greater than the sum of those obtained with each extractant, This increased rate is also expressed satisfactorily by the diffusion steps as in the case of DNNSA micelles, The synergistic effect on the extraction rate for beryllium is explained by an increase in the distribution ratio of the metal caused by inclusion of D2EHPA molecules into DNNSA micelles, A larger acceleration effect is observed for aluminum, suggesting reactive extraction by D2EHPA at the micro interface of reverse micelles in addition to the bulk interface.