In order to alter the physical/chemical characteristics of multi-walled carbon nanotubes (MWNTs) we modified them by different organic reactions (Diels-Alder and Sand-Meyer reaction, oxidation) and their d properties were characterized by thermogravimetry/mass spectrometry, photoelectron spealterectroscopy, and nuclear magnetic resonance spectroscopy, as well as by dispersion. The results proved that, depending on the groups built in the MWNTs, the modified carbon nanotubes are more dispersible either in polar or apolar solvents and the suspensions are stable for long time. The presence of the substituents in the MWNTs was proved by methods listed above, e.g. high concentration of sulfur was detected when SO3H groups were inserted onto the MWNTs. The enhanced thermal stability of the modified carbon nanotubes allows their further application.