Magnetic Fe3O4/chitosan nanoparticles (MFe3O4/CS NPs) were in situ prepared successfully by a simple reduction-precipitation method through partial reduction of Fe3+ with sodium sulfite, precipitation with ammonia at room temperature, and then cross-linking with epoxy chloropropane. The nanoparticles were characterized by HRTEM, BET, XRD, FT-IR, TGA and VSM. MFe3O4/CS NPs were quasi-spherical or ellipsoidal in shape and about 7 nm in average diameter, exhibiting superparamagnetic properties with a saturation magnetization value of 17.1 emu/g. The adsorption behaviors of MFe3O4/CS NPs towards reactive brilliant red X-3B (X-3B) were investigated, including pH effect, adsorbent dosage, adsorption equilibrium and adsorption kinetics. It was found that the adsorption performances of X-3B on MFe3O4/CS NPs were strongly dependent on both initial pH and the adsorbent dosage. The adsorption equilibrium data fitted well with the Langmuir isotherm model and the adsorption process followed the pseudo-second-order kinetic model. Thermodynamic parameters such as free energy (Delta G degrees), enthalpy (Delta H degrees), and entropy (Delta S degrees) indicated that the adsorption was spontaneous and endothermic. Besides, the recycling experiments showed good reusability of MFe3O4/CS NPs as adsorbents. (C) 2014 Elsevier B.V. All rights reserved.