The metal ion extractant (HDEHP) that is commonly employed in liquid membrane extraction has been shown to stabilise Ag and Cu nanoparticles in oil media of size 5 and 10 nm, respectively. The particle are formed by reduction of the metal salts of HDEHP that are oil soluble and shown by SANS to form small reversed micelles of radius similar to 1 nm. The extractant is also effective at stabilising particles of similar size in an oil phase when the metal ion (e.g., AgNO3) is reduced in a coexisting aqueous phase. (C) 2008 Elsevier Inc. All rights reserved.