This paper describes the results of detailed studies on the roles of two components used as carrier for the transport of Cu(II) through the permeation liquid membrane (PLM): lauric acid (LAH) and NN-didecyl-diaza-18-crown-6ether (22DD). An overall mechanism of Cu(II) transport is proposed, based on a number of results reported in this and previous papers. The structure of the active forms of Cu(II) species participating in each step of the transport is proposed on the basis of complementary spectroscopic (UV/vis, NMR, FTIR) studies. This, together with the PLM transport experiments at high (5 x 10(-5) M) and low (5 x 10(-7) M) copper(II) concentration allowed us to identify the conditions under which the precipitation of copper(II) laurate on the membrane surface can be avoided in order to optimize the PLM procedure of speciation. (C) 2008 Elsevier B.V. All rights reserved.