XRD and XPS analyses revealed that a Fe(NO3)(3)center dot 9H(2)O layer formed outside gamma-Fe2O3 particles when Fe3O4 nanoparticles were treated with ferric nitrate. The particle density differed for untreated and treated particles and was not uniform for the latter. The specific saturation magnetization of both treated and untreated particles was used to estimate the thickness of the Fe(NO3)(3)center dot 9H(2)O layer and the average density of the treated particles. The density of the treated particles was used to calculate the density of ferrofluids of different particle volume fractions. These values are in agreement with measured results. Therefore, the particle volume fraction can be designed to synthesize acid ionic ferrofluids based on Fe3O4 nanoparticles using Massart's method. (C) 2010 Elsevier B.V. All rights reserved.