Multi-walled carbon nanotubes (MWCNT) were functionalized with a variety of chemical groups by reaction of p-substituted anilines (R-Ph-NH2) in the presence and absence of isopentyl nitrite used for the in situ generation of diazonium species. All materials were characterized by X-ray photoelectron spectroscopy, thermogravimetry and infrared spectroscopy. In the presence of isopentyl nitrite, the extent of functionalization was high and nearly independent on the amount of isopentyl nitrite and on the aniline substituents (R = F, Cl, I, NH2, NO2, OH, COOH, COOEt and Et). Unexpectedly, the functionalization of MWCNT with anilines bearing electron withdrawing groups was also observed in the absence of isopentyl nitrite. In the case of OH-Ph-NH2, the reaction leads to MWCNT with the highest degree of functionalization and this can be considered as a new and efficient methodology for CNT functionalization with phenol groups. The overall reaction mechanism is discussed for both reaction conditions: confirmation of a radical chain mechanism was obtained for the reaction performed in the presence of isopentyl nitrite, while the formation of stabilized dipolar intermediate species seems to be involved in the absence of isopentyl nitrite. The materials with the highest degree of functionalization showed very good dispersions in acetonitrile even after 1 month.