A two-step process for producing opened carbon multi-walled nanotubes (MWNTs) of high purity is presented. The tips are attacked and opened selectively by concentrated nitric acid, while in the second step gazification under CO2 at 525 degrees C eliminates fully the debris of disordered carbon coming from the tips destruction. The graphitic layers of the obtained MWNTs are not altered by the treatment. The average diameter slightly decreased from 10 to 15 nm for the pristine nanotubes to 8-12 nm for the opened nanotubes, due to the mild oxidation under CO2 which contributes to erode uniformly the outer layers of nanotubes. (c) 2005 Elsevier B.V. All rights reserved.