Maghemite (gamma-Fe2O3) nanoparticles dispersed in water and forming a magnetic fluid (MF) were characterized by Raman and FT-NIR spectroscopy and were also investigated by surface-enhanced Raman spectroscopy (SERS) and cyclic voltammetry (CV). The FT-NIR results have shown that magnetite (Fe3O4) is not present on the particle as proposed by other investigators. The SER spectra and CVs results have shown that the MF nanoparticles adsorbed on the Ag electrode undergo four reduction processes. At -0.14 V, a positively charged non-stoichiometric Fe(III) oxy-hydroxide surface layer is reduced to Fe3O4. At -0.62 and -0.88 V, delta-FeOOH and gamma-Fe2O3, respectively, are reduced to Fe3O4. Finally, at potentials more negative than -1.1 V, Fe3O4 formed in the previous processes is reduced to Fe-0. In the reverse voltammetric scan the SERS spectra are quite different showing that the process is irreversible, since during oxidation of the film Fe(OH)(2) and Fe(OH)(3) have also been characterized. The SERS and electrochemical results lead us to conclude that the nanoparticles are composed of a gamma-Fe2O3 (maghemite) nuclei that present a surface layer containing delta-FeOOH and a positively charged non-stoichiometric Fe(III) oxy-hydroxide surface layer. (c) 2007 Elsevier B.V. All rights reserved.