In this study, the catalytic reduction of 4-nitrophenol by heterostructured Au-Fe3O4 nanocatalysts using NaBH4 as the reducing agent was investigated under various environmental conditions. The electron behaviors at the interface of Au and Fe3O4 nanoparticles were examined to elucidate the reaction mechanisms for 4-nitrophenol reduction. The transmission electron microscopic images show that the average particle size of Au-Fe3O4 heterostructures increases slightly from 14 to 18 nm after phase transfer from oil phase to aqueous solution. The X-ray photoelectron and X-ray absorption near edge spectroscopic results show the electron flow from Au seeds to Fe3O4, resulting in the formation of positively charged Au surface to accelerate the catalytic reduction efficiency and rate of 4-nitrophenol. In addition, the reduction of 4-nitrophenol is a surface-mediated reaction and the catalytic efficiency and rate of 4-nitrophenol is highly dependent on the initial 4-nitrophenol concentration, pH, and reaction temperature. The increase in pH lowers the reduction efficiency and rate of 4-nitrophenol and a 2.4-fold decrease in the pseudo-first-order rate constant is observed when pH increases from 5 to 9. In addition, the Au-Fe3O4 nanocatalysts show a good separation ability and reusability which can be repeatedly applied for complete reduction of 4-nitrophenol for at least six successive cycles without the loss of morphology and saturation magnetization. Results obtained in this study clearly demonstrate that the Au-Fe3O4 heterostructures are excellent nanocatalysts which can be applied in heterogeneous catalysis, water treatment, and green chemistry. (C) 2014 Elsevier B.V. All rights reserved.