The exhaustive methylation of the three isomers of the icosahedral carboranes 1,2-, 1,7-, and 1,12-dicarbadodecaborane(12) (ortho-(1), meta-(2), and para-carborane (3)) under Friedel ± Crafts conditions using MeI/AlCl 3 is described. Regioselectively ortho-and meta-carborane are octamethylated providing 4,5,7,8,9,10,11,12-octamethyl-1,2-dicarba-closo-dodecaborane(12) (6) and 4,5,6,8,9,10,11,12-octamethyl-1,7-dicarba-closo-dodecaborane(12) (8) in high yield. Their BH vertices adjacent to both CH vertices remain unmethylated. Paracarborane can be quantitatively converted to its deca-B-methyl derivative 4.