In the purification procedure of catalytically prepared carbon nanotube samples, the oxidative removal of amorphous carbon was investigated. Oxidation was carried out by KMnO4, H2O2, O-3 or HClO4. The selectivity of the amorphous carbon burning reaction was studied as a function of the reaction temperature by transmission electron microscopy. The advantages of the different methods are as follows: although oxidation by KMnO4 in an acidic suspension provides nanotubes free of amorphous carbon, and can easily be controlled, it must be followed by filtration and cc HCl treatment in order to dissolve the MnO2 formed during the reaction. Oxidation with hydrogen peroxide does not give undissolved residue on the surface of the tubes. Ozone treatment produces gaseous by-products, but requires higher reaction temperature. This method is more difficult to control. During oxidation, competitive oxidation always takes place: carbon nanotubes and amorphous carbon react simultaneously. While amorphous carbon can be attacked from any direction, carbon nanotubes can be oxidized only from the ends.