The extraction and transport of Cd(II) and Pb(II) in two different membrane systems (SLM and PIM) using Kelex 100 as carrier was studied, proposing the corresponding chemical models of transport. A two-species transport model is proposed for Cd(II), according to solvent extraction (SX) data. Experimental SLM permeabilities are 9.7 x 10(-5) ms(-1), while measured PIM permeabilities are 5 x 10(-5) ms(-1). Values for the aqueous boundary layer thickness and for the diffusion coefficient of the metal cation-carrier complexes in the membrane phase were calculated from numerical fitting of experimental data using the proposed transport models. A highly selective Pb(II) separation was achieved in PIM systems based on the nature of the chemical equilibria involved in Cd(II) and Pb(II) membrane transport.