A magnetic chitosan (CS) composite material was synthesized successfully by a new procedure, ammonia fumigation, which is suitable for large-scale production. CS dissolved in an aqueous acetic acid solution was mixed with [Fe(SO4)2]2− and [Fe(SO4)2]− ions to obtain a light yellow precipitate, which was then exposed to ammonia vapor to prepare the composite adsorbent product. The microstructure of the composite was studied by scanning electron microscopy, transmission electron microscopy, X-ray diffraction, and Fourier transform infrared spectroscopy. The results indicate that Fe3O4 particles 3–7 nm in size were formed in situ and were stabilized in the CS matrix. The composite adsorbent showed superparamagnetic properties with a saturation magnetization of about 11.0 emu/g. Moreover, the composite material displayed good adsorption of methyl orange and copper ions. © 2013 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2014, 131, 40057.