The transport of thiourea in acidic or neutral chloride solutions through a polyvinyl chloride (PVC) membrane containing immobilized Aliquat 336 was studied experimentally. No chemical interaction between thiourea and Aliquat 336 was observed. The acidity constant of thiourea at 1.0 ionic strength was 13.7 and its distribution constant fur acidic solutions and near Aliquat 336 was 4.20. A mathematical model assuming ideal mixing in the solutions on both sides of the membrane and Fickian diffusion in the membrane was developed and used for the determination of the diffusion coefficient or thiourea in membranes with 30, 40, and 50 wt% Aliquat 336 content. The corresponding values are estimated to be 5.5 x 10(-13) m(2)s(-1), 1.2 x 10(-12) m(2)s(-1) and 3.5 x 10(-12) m(2)s(-1), respectively. The transport properties of PVC membranes with other plasticizers [bis(1-butylpentyl) adipate, dioctylphenyl phosphonate, and n-decanol] were compared to the Aliquat 336/PVC membrane.