The formation of the collapsed carbon nanotubes (CNTs) in a chemical vapor deposition process is studied by intentionally adding some toxic materials (sodium carbonate, sodium phosphate, and calcium chloride) of the catalyst into the CNTs growth system. The distorted CNTs and fully collapsed CNTs are observed as compared to the pristine CNTs with relatively perfect morphology. The yield of carbon decreases and the defects inside the CNTs increase with the increasing addition of these poisonous materials. The characterization by Raman spectroscopy shows that the length of in-plane carbon of these carbon products treated by high temperature decreases nearly linearly with the increasing of the amount of poisonous materials. The formation of the collapsed nanotubes is possibly attributed to the decreased yield of carbon and the significant difference between carbon concentrations when the carbon diffuses from the poisoned part and the unaffected part of metal particles to the precipitation interface for the formation of nanotubes. (C) 2003 American Institute of Physics.