Fe 3 O 4 -multi-walled carbon nanotubes (Fe 3 O 4 -MWCNTs) hybrid materials were synthesized by a solvothermal process using acid treated MWCNTs and iron acetylacetonate in a mixed solution of ethylene glycol and ultrapure water. The materials were characterized using X-ray powder diffraction, scanning and transmission electron microscopy, X-ray photoelectron spectroscopy, and vibrating sample magnetometry. The results showed that a small amount of water in the synthesis system played a role in controlling crystal phase formation, size of Fe 3 O 4 , and the homogeneous distribution of the Fe 3 O 4 nanoparticles deposited on the MWCNTs. The Fe 3 O 4 nanoparticles had diameters in the range of 4.2–10.0nm. They displayed good superparamagnetism at room temperature and their magnetization was influenced by the reaction conditions. They were used as a Fenton-like catalyst to decompose Acid Orange II and displayed a higher activity than nanometer-size Fe 3 O 4 .