This paper describes the effect of four counter anions (CH3COO-, Cl-, NO3-, ClO4-) of the trioctylmethylammonium (TOMA) cation on the rate of solvent extraction of thiourea and its transport across poly(vinyl chloride) (PVC)-based polymer inclusion membranes (PIMs). The membranes also contained 2-nitrophenyl octyl ether (NPOE) as the plasticizer while chloroform was used as diluent in the solvent extraction studies. It is demonstrated that the counter anion affects substantially the rate of membrane transport and the degree of extraction follows the order: CH3COO- > Cl- > NO3- >> ClO4-. The transport rate is negligible for the perchlorate anion. This order is consistent with thiourea interacting with the counter anion through hydrogen bonding to form a heteroconjugate anion. A mathematical model of the membrane transport of thiourea assuming ideal mixing in the feed and receiver solutions and Fickian diffusion of the heteroconjugate anion has been developed and fitted to the experimental transport data. This has allowed the determination of the diffusion coefficient of the heteroconjugate anion (5.28 x 10(-11) m(2) s(-1)) and the thiourea membrane extraction constants (CH3COO-: 3.41; Cl-: 2.61 x 10(-1); NO3-:2.26 x 10(-2) m(3) mol(-1)) which follow the same order as the solvent extraction constants (CH3COO-: 2.42; Cl-: 6.63 x 10(-1); NO3-: 4.26 x 10(-1); ClO4-: 8.31 x 10(-2) m(3) mol(-1)). The exceedingly low ability of PIMs containing TOMA-ClO4 to extract and transport thiourea suggests that these membranes can potentially overcome the difficulties in the PIM separation of gold(III) which are associated with the counter transport of thiourea. (C) 2009 Elsevier B.V. All rights reserved.