Praseodymium and holmium have been chosen as representative elements of light and heavy lanthanides, respectively, to characterize the extraction reactions of these elements that are responsible for their separation through supported liquid membranes (SLM) from nitrate media by using di-(2-ethylhexyl) phosphoric acid (D2EHPA) in kerosene as extractant. in the conditions studied, it has been demonstrated, the formation of two species that can be described as Ln(NO3)A(2)(HA)(3) and LnA(3)(HA)(2), where Ln represents the lanthanide cation and HA the acidic extractant D2EHPA. These results have been obtained by using both graphic and numeric treatments of solvent extraction data. The first one was based on the slope analysis, and the second one, on the use of LETAGROP-DISTR program. The formation constants and relative predominance of each species were calculated. A mechanism for the formation of given species was proposed.