We describe the behavior of iron and cobalt catalysts on silicon substrates in chemical vapor deposition (CVD) of carbon nanotubes. Nanoparticles of iron and cobalt exhibited a melting point drop in methane ambient. Nanoparticles present after nanotube growth are identified as Fe₃C and Co₃C, for iron and cobalt, respectively. These findings indicate that a eutectic compound of metal and carbon is formed in the methane ambient, resulting in the phase separation of graphite (nanotubes) as the carbon uptake in the catalyst melt increases. This supports the vapor-liquid-solid mechanism for the nanotube growth by CVD. Iron- or cobalt-silicide formation causes poisoning of the catalysts. However, the coexistence of oxygen due to native oxide on the silicon surface or the metal surface causes formation of a SiO₂ base that can prevent silicidation of iron particles.