Formation of cobalt-encapsulating carbon nanotubes/nanoparticles from the decomposition products of CO(CO)(3)NO was investigated by carrying out the reaction in a specially arranged closed cell at different temperatures. Multiwalled carbon nanotubes along with carbon-encapsulated cobalt nanoparticles were generated via a catalytic carbon monoxide disproportionation reaction over the in situ formed cobalt nanoparticles on an MgO support. The carbonaceous materials were separated from the product by a simple acid-treatment method. Structure and composition of the products were characterized by X-ray diffraction (XRD), transmission electron microscopy, and thermogravimetric analysis. It was observed that nanotube fort-nation is favored at 1000 degreesC, whereas lower temperatures produced mainly nanoparticles with or without the encapsulated cobalt. The average outer diameter of the nanotubes was 26 nm with an inner diameter of 15 nm. As revealed from XRD, the encapsulated cobalt particles were in their high-temperature fcc phase and were present at the tip of the nanotubes and inside the nanoparticles. Magnetization measurements showed that the encapsulated cobalt particles are ferromagnetic in nature and the saturation magnetization (M-s) and coercive force (H-c) are dependent on the react. ion temperature.