For the purpose of developing new technologies of extraction and separation of metals, we have investigated the way of using microcapsules. In this study, the microcapsules containing several organophosphorus compounds as extractants (HR) were prepared by in situ polymerization technique and their characteristics were investigated. The extractant held within microcapsules was not leaked out at pH below 4, hence the microcapsules are useful for the extraction of cationic metal ions. In this study, we investigate the extraction behavior of gallium and indium as objective metal sources. The extraction equilibria were clarified by a slope-analysis method. Those for gallium and indium using the microcapsules containing di(2-ethylhexyl)phosphoric acid (D2EHPA) and 2-ethylhexylphosphonic acid mono-2-ethylhexyl ester (EHPNA) were determined as follows: Ga3+ +2(HR)(2) reversible arrow GaR3 HR + 3H(+) In3+ + 5/2(HR)(2) reversible arrow 2HR + 3H(+) The extraction equilibrium for the microcapsule containing diisostearyl-phosphoric acid (DISPA) was determined as follows: (Ga,In)HSO42+ + 5/2(HR)(2) reversible arrow (Ga,In)HSO4R2 3HR + 2H(+) The extraction equilibrium constants and separation factors were also determined for each system. The maximum adsorption capacities of the metals in the microcapsule containing EHPNA were determined with the Langmuir isotherm model and determined as 4.49 x 10(-4) and 3.22 x 10(-4) mol/g-MC for gallium and indium, respectively.