Single phase TmB 12, YbB 12 and LuB 12 were successfully obtained by the arc-melting and floating zone methods. The XPS measurements of these compounds have indicated that (1) only YbB 12 is a valence fluctuating compound with the valency of +2.9 at room temperature, (2) the correlation energy between Yb 2+ and Yb 3+ ions is about 6.0 eV, (3) Tm and Lu ions in the dodecaborides are in a trivalent state, and (4) the binding energy of the 4f level in TmB 12 and LuB 12 is 5.0 and 7.7 eV, respectively.