In this work a successfully applied method is reported for the preparation of stable colloidal cobalt-based oils. The two-step process comprises thermolysis of a Co 2(CO) 8-toluene solution in the presence of Na-AOT or lauroylsarcosin oleic acid followed by an exchange step for the carrier liquid. In this step a second surfactant compatible with the oil carrier was used leading to a double-layer coating of the Co particles. Magnetic oil-based liquids with a saturation magnetization up to 120 mT could be prepared this way. The sedimentation stability in the presence of a strong magnetic field and the oxidation resistance are excellent.