Carbon nanotubes, which can alternatively be described as cylindrically rolled graphitic carbon sheets, are new and promising materials that have evolved from the discovery of fullerenes and related materials [1±3]. They may have signi®cant potential in advanced electrical and mechanical applications. For instance, the modi®cation of carbon nanotubes by ®lling them with a metal species opens up several possible uses because the properties of the modi®ed ®bers can be very different from the original carbon material. Encapsulation [4, 5] might have interesting effects on the properties (such as superconductivity or magnetism) of the interior phase. Transition metal carbides [6±9] are characterized by several attractive properties, including high melting point, strong chemical resistance, extreme hardness, strength at high temperature and even superconductivity. For instance, the melting points of TiC, NbC and TaC are 3067, 3600 and 3983 8C, respectively; TaC has the highest melting point known for any material. Polycrystalline samples of the carbides can be prepared by powder metallurgy at the following temperatures: TiC (1700±2100 8C), NbC (1300±1400 8C) and TaC (1300±1500 8C). In this study, nanoparticles of TiC, NbC and TaC were synthesized by a solid-state reaction using carbon nanotubes. The structure of the resulting materials was analyzed by X-ray diffraction (XRD) and high-resolution scanning electron microscopy (HRSEM). Multilayer carbon nanotubes (.90% purity) synthesized by metal-catalyzed growth using ethylene were prepared as precursors. Carbon nanotubes; transition metals (titanium powders (,325 mesh, 99.99%), niobium powders (,60 mesh, 99.8%) and tantalum powders (,325 mesh, 99.98%)); and a small amount of iodine (,14 mesh, 99.999%), which was used as a transport agent, were packed in quartz ampoules having a diameter of 5 mm. The molar ratio of carbon in the nanotubes to transition metals to iodine was 10:10:1. The ampoules were evacuated and back-®lled with argon (99.99%) three times. The samples were then heated in the quartz ampoules at 1000 8C or 1100 8C for 168±215 h. In this reaction, iodine seems to act as a transport agent, as shown in this chemical equation [10] CM I4 or 5(gas) MC (2 or 5=2) I2 (1)