The calculation of the initial flux (J(0)) of a polymer inclusion membrane (PIM) is an important task in characterizing its transport performance. This article outlines the difficulties in reliably determining J(0) of a model PIM system for the extraction of U(VI) from its sulfuric acid solutions. The membrane consists of poly(vinyl chloride) and di-(2-ethylhexyl)phosphoric acid (D2EHPA). Several approaches for determining J(0) are compared and most reliable results are achieved when the transient U(VI) solution concentration is fitted with an exponential decay function (Psi = a(1) + a(2)e(-ast)), the first derivative of which ((d Psi/dt)(t=0)) is used to calculate J(0). The use of a quantity referred to as dimensionless initial flux which is independent of the solution volume, membrane surface area and initial concentration of the solute is proposed. It will allow a more meaningful comparison of PIM transport properties on the basis of extraction and transport data obtained under different experimental conditions. The conclusions derived in this study are applicable to other membrane separation systems such as those incorporating supported liquid membranes. (c) 2013 Elsevier B.V. All rights reserved.