Bilayer oleic acid-coated Fe3O4 nanoparticles can be applied in more areas than single layer oleic acid-coated ones because they can be well dispersed not only in nonpolar carrier liquids but also in polar carrier liquids, while the single layer oleic acid-coated ones can be dispersed only in nonpolar carrier liquids. Therefore, it is of significance to characterize the surface structure of bilayer and single layer oleic acid-coated Fe3O4 nanoparticles. However, there existed a discrepancy in the characteristic FTIR spectrum of the secondary layer in bilayer oleic acid-coated Fe3O4 nanoparticles. The goal of this paper was to resolve the discrepancy through using FTIR and TGA together with dispersibility to characterize the surface structure of bilayer and single layer oleic acid-coated Fe3O4 nanoparticles. The results showed that the band at 1710 cm(-1) was the characteristic band of the secondary layer in bilayer oleic acid-coated Fe3O4 nanoparticles. It can be used to distinguish whether the oleic acid-coated Fe3O4 nanoparticles are bilayer or not. (C) 2009 Elsevier B.V. All rights reserved.