Heat-treated catalytic multiwall carbon nanotubes (MWNTs) were intercalated by K andFeCl3in vapor phase, using the two-bulb technique. A first stageKC9intercalation compound was formed with potassium. After elimination of potassium, the tubular morphology is still preserved showing that intercalation is a reversible phenomenon. In the case ofFeCl3,the saturated compound is less rich than with graphite. However, well defined in planehkbands prove the intercalation. Due to the position of the002line at 0.345 nm, it is likely that intercalation is incomplete and that the material is a mixture of intercalated and non intercalated zones. A model of catalytic nanotubes is presented which accounts for the reversibility of the intercalation reactions. ©1998 American Institute of Physics.