Poly(vinyl chloride) (PVC)-based membranes containing Aliquat 336 as the extractant are used in a study of the extraction of Cd(II) and Cu(II) from HCl solutions. Factors influencing the extraction with these membranes such as the thickness, the composition and the nature of the surface are studied. A mathematical model for the extraction of Cd(II) is used which is based on an extraction mechanism governed by both the chemical kinetics and the membrane diffusion processes. Fitting the model to the extraction data provides values of the diffusion coefficient and the kinetic rate constants of the complexation reaction between the extractant and the extracted species.