The formation of gold (Au) nanocrystalline films by reduction at the liquid/liquid interface has been investigated: three product fractions have been characterized, via electron and atomic force microscopy, X-ray photoelectron and UV-vis absorption spectroscopy, and X-ray diffraction. The mechanism suggested is deposition and self-assembly of the Au nanoparticles (NPs) at the liquid/ liquid interface occurring in close association with the adsorption of capping ligands obtained from the starting materials and the cleavage of the reducing agent, tetrakis(hydroxymethyl)phosphonium chloride (THPC). NPs can also be assembled at the liquid/liquid interface by adsorption from one of the adjacent solution phases. This parallel route is also followed here to demonstrate that particle formation can occur either via route (i) interfacial reaction or (ii) bulk reaction followed by selfassembly. © 2009 American Chemical Society.