Homogeneous carbon-based materials were prepared for endohedral functionalization starting from carbon nanotubes (CNT) samples mainly containing double-walled and very-thin multi-walled CNTs. Comprehensive structural characterization of end-opened and shortened CNTs, which were prepared under mild oxidative conditions using dilute aqueous H2O2 solutions, is reported. 90% of the 400 measured nanotubes were shorter than 1 μm with an average value of about 500 nm. TEM observations have confirmed the shortening and opening of the end caps. TGA analysis and Raman spectroscopy showed a reduced impurity content after the oxidation treatment.