The extraction of iron(III) from acidic sulfate solutions by bis(2-ethylhexyl)phosphoric acid (HDEHP) is investigated by using PENRECO(R) 170 ES as a diluent. PENRECO(R) 170 ES is new diluent which offers advantages such as improved solvency power, more complete phase disengagement and reduced losses in aqueous streams, with reductions of over 50% in diluent usage after 1 year, compared with conventional paraffinic diluents. The chemical analyses performed in the present work suggest that such properties arise, at least in part, from the presence of a series of hydrophobic branched alcohols in its composition (at least 0.6 mol dm(-3)). In spite of the solvation effects due to these alcohols, HDEHP is dimeric in this diluent and, in the presence of an excess of HDEHP, the extraction of iron(III) takes place according to the classical equation: Fe3+ + 3(H2L2) double left right arrow (FeL3.3HL) + 3H(+) with K-ex = 10(5.7+/-0.2) (at I = 1mol dm(-3)). Such a value of K-ex is similar to that reported for pure hexane, which shows that the presence of long chain alcohols in PENRECO(R) 170 ES has no perceptible influence on the thermodynamics of iron(III) extraction by HDEHP. The extraction of iron(III) by HDEHP in PENRECO(R) 170 ES is slightly more rapid than in kerosene, which indicates that the molecules of alcohols constituting PENRECO(R) 170 ES have no negative effect on the kinetics of metal extraction although they compete with the extractant molecules for adsorption at the liquid-liquid interface. Stripping of iron(III) from loaded organic solutions by sulfuric acid is easy and rapid (95% equilibrium reached within 2 min) when HDEHP is used at moderate concentrations (typically 0.1 mol dm(-3)). At higher HDEHP concentrations, stripping is difficult and incomplete, as found previously with other diluents. Thus, PENRECO(R) 170 ES is interesting in its ability to overcome some of the physical problems encountered in liquid-liquid operations, but its use does not modify significantly the chemistry of iron(III) extraction by HDEHP. 2002 Society of Chemical Industry