The relative reactivities of the closo carboranes C2BnHn+2 (n = 4, 5, 8), and some of their derivatives, toward electrophilic reagents of the type RX/AlCl3 (RX = CH3Cl, C2H5Cl, Cl2, Br2) are reported from competition studies. Among the three parent carborane compounds, closo-2,4-C2B5H7 is the most reactive toward an electrophilic type of substitution. Alkyl substituents on closo-2,4-C2B5H7 enhance the reactivity of the compound toward an electrophilic substitution, whereas halogen substituents decrease the reactivity. However, in the closo-1,6-C2B4H6 system, the reactivity of the chloro-substituted compound, 2-Cl-1,6-C2B4H5, toward an electrophilic substitution is greater, at the 4-position, than that of the parent carborane. The nature (halogen or alkyl) and cage position of a substituent on closo-2,4-C2B5H7 appear to have little or no influence on the site of “electrophilic” substitution.