Concentrated phosphoric acid issued from the "wet process" contains at least 19 different contaminants, among which are metal elements, that must be removed to obtain purified acid valuable in many applications. Nanofiltration of concentrated inorganic acids is an emerging application for NF that could be used for purification of raw phosphoric acid. Nevertheless, an in-depth study of transfer mechanisms is required in order to properly understand the observed retentions of metal species during NF of raw phosphoric acid and to be able to propose a better management of the process for further purification. NF mechanisms concerning metal ions in diluted aqueous solutions are extensively studied, but up to now limited literature data deal with concentrated acids. This paper is a contribution to a better understanding of transfer of low amount of metal species in concentrated phosphoric acid, the concentration of which can be as high as 5.9 mol L-1. The study focuses on the transfer of iron that is the main metal contaminant of the raw acid. To achieve this goal, model solutions without and with iron chloride (18.6 x 10(-3) mol L-1 FeCl3) in H3PO4 (0.12-1.2-5.9 mol L-1) are filtered in a stirred dead-end cell using a MPF34 membrane. Retentions of phosphoric acid, that can be in neutral (H3PO4) and charged (H2PO4-) form, is discussed. Dealing with iron-phosphate chelates, Fe(H2PO4)(4)(-) is the predominant one whereas two cations can sometimes exist: Fe(HPO4)(+) and Fe(H2PO4)(2+). The divalent cation is in significant amount only in 0.12 mol L-1 acid concentration medium. Retentions of iron chelates by the NF membrane are determined from both the mass balance of iron and their dissociation constants. For 5.9 and 1.2 mol L-1 media, retentions decrease in the following order: Fe(H2PO4)(4)(-) > H3PO4 > H2PO4- > H+.Cl- At high acid concentration chloride retention is negative suggesting that electrostatic interactions can be involved in transfer mechanism at both 5.9 and 1.2 mol L-1. (C) 2012 Elsevier B.V. All rights reserved.