We present here a simple and effective strategy, called the vacuum nano-casting route (VNR), for fabricating hollow mesoporous spheres with magnetic cores. The loading amount of magnetic cores and saturation magnetization value can be easily tuned by changing the concentration of iron nitrate solution used in the synthesis procedure. After modification with 1,4-bis(triethoxysilyl)propane tetrasulfide (BTESPTS), the composite could be used as a highly selective absorbent of Hg 2+ , and was conveniently separated by an external magnetic field. The composites are characterized by X-ray diffraction, N 2 sorption isotherms, transmission electron microscopy, energy-dispersive spectroscopy, vibrating-sample magnetometry, Fourier-transform infrared spectroscopy and inductively coupled plasma atomic emission spectroscopy.