The action of AlCl3on room-temperature tetrachloromethane solutions ofanti-B18H22(1) results in a mixture of fluorescent isomers, 3,3′-Cl2-B18H20(2) and 3,4′-Cl2-B18H20(3), together isolated in a 76% yield. Compounds2and3are capable of the stable emission of blue light under UV-excitation. In addition, small amounts of other dichlorinated isomers, 4,4′-Cl2-B18H20(4), 3,1′-Cl2-B18H20(5), and 7,3′-Cl2-B18H20(6) were isolated, along with blue-fluorescent monochlorinated derivatives, 3-Cl-B18H21(7) and 4-Cl-B18H21(8), and trichlorinated species 3,4,3′-Cl3-B18H19(9) and 3,4,4′-Cl3-B18H19(10). The molecular structures of these new chlorinated derivatives of octadecaborane are delineated, and the photophysics of some of these species are discussed in the context of the influence that chlorination bears on the luminescence ofanti-B18H22. In particular, this study produces important information on the effect that the cluster position of these substitutions has on luminescence quantum yields and excited-state lifetimes.