We have studied the stoichiometry of magnetite nanoparticles using three spectroscopic techniques: Mossbauer, photoacoustic and ferromagnetic resonance (FMR). By varying the weight ratio of the Fe precursor to the reducing agent (sodium acetate) and a post-synthesis annealing, we were able to synthesize samples with different amounts of Fe vacancies, from stoichiometric Fe3O4 to gamma-Fe2O3. By synthesizing magnetite in the presence of zeolite we obtained nanoparticles within the 3-10 nm diameter range. The spectroscopic results show that there is a correlation between the amount of Fe vacancies and (i) the optical absorption and (ii) the g-values from the Electron paramagnetic resonance EPR spectra of the nanoparticles. (C) 2016 Elsevier B.V. All rights reserved.