The halogenation of carbon nanotubes is of great interest, as it allows the modification of the tube properties or it can serve as primary functionalization for a subsequent covalent fixing. Large degrees of fluorination can be reached although chlorine fixing is preferred, if halogenation is used as primary functionalization, as it is a better leaving group. Here, we report an extensive chlorination of multiwalled carbon nanotubes (MWCNTs) by I2Cl6. The reaction has been studied in acetic acid (AC) and in carbon tetrachloride (TC). The influence of the solvent is critical, as the functionalization is not relevant when the reaction is carried out in AC, while it reaches up to 13 atom % when the solvent is TC. Moreover, chlorine functionalization occurs in defects and borders of the tubes when the solvent is AC, while chlorine is largely fixed on the tube walls when TC is the reaction solvent. These different results are due to iodine trichloride coming out in different chemical species depending on the solvent. The correlation of the TPD and XPS spectra has allowed assignment of the fixed chlorine atoms to five chemical environments. None of the samples show intercalation of chlorine as deduced from X-ray diffraction data.