Fully chlorinated carborate and dodecaborate cages such as [CHB11,Cl-11](-) and [Me3NB12Cl11](-) are prominent examples of valuable and chemically rather inert weakly coordinating anions. While both anions can be obtained by chlorination of the precursors [CH12B11](-) and [H3NB12H11](-) with SO2Cl2 followed by methylation for the synthesis of [Me3NB12Cl11](-), best results were found using photochemical chlorination with SO2Cl2 for [CH12B11](-) and thermal chlorination with SO2Cl2 for [H3NB12H11](-). The hexachlorinated anion [n-Pr3NB12H5Cl6](-) was formed readily within 30 min by chlorination of [n-Pr3NB12H11](-), but attempts to synthesize isopropyl-substituted ammonio-dodecaborates with a higher chlorination number resulted in the formation of mixtures and partial decomposition. The silver and trityl salts of the anions [CHB11Cl11](-), [Me3NB12Cl11](-), and [n-Pr3NB12H5Cl6](-) as well as the contact ion-pair [Et2Al][Me3NB12Cl11] were also prepared, and the compounds [Ag(NCMe)][Me3NB12Cl11], [Et2Al][Me3NB12Cl11], and [Et4N][i-Pr3NB12H5Cl6] were also characterized by X-ray crystallography.