The equilibria of aluminum and beryllium extraction from sulfate media are using single systems of bis(2-ethylhexyl) phosphoric acid (D2EHPA) and dinonylnaphthalene sulfonic acid (DNNSA), and also the mixed system of these extractants have been studied. In the single D2EHPA system, the extraction species are found to be AIR(3) . HR and BeR2 . 2HR and separation of the two metals is satisfactory, although the extraction rate is rather slow. In the single DNNSA system, the DNNSA is found to form reverse micelles at concentrations greater than 10(-5) M. Separation is poor and the extraction mechanism of the metal ions is influenced by sodium ion. The presence of D2EHPA increases the critical micelle concentration (CMC) of DNNSA. For DNNSA concentrations below the CMC, the extraction mechanism in the mixed system is identical to that for the single D2EHPA system. For DNNSA concentrations greater than the CMC in the mixed system, the distribution ratio increases and separation of the metals becomes more difficult. This type of synergistic effect appears to be caused by inclusion of D2EHPA molecules in the DNNSA micelles.