The mechanism of lanthanum transport through a supported liquid membrane is presented. The membrane consisted of a Teflon millipore membrane with a kerosene solution of di(2-ethylhexyl) phosphoric acid (HDEHP) as a mobile carrier held within the pores by capillary forces. Interposing the liquid membrane between two aqueous solutions with different pH values, lanthanum was transported and concentrated from the high pH solution to the low pH solution across the liquid membrane. The effects of HDEHP concentration in the membrane solution and of the lanthanum concentration and pH in the aqueous phases on the permeation rates of lanthanum were investigated. It was observed that the permeation rates decrease drastically by the addition of surfactant to the membrane phase. The permeation rates of lanthanum can be explained by a permeation model which includes the extraction and the stripping reaction at the membrane interfaces and the diffusion process of the complex formed between lanthanum and HDEHP through the membrane in addition to the diffusion process of lanthanum in the aqueous film adjacent to the membrane.