Light lanthanide dodecaborides, RB12 (R = Pr and Ce), were synthesized from a stoichiometric mixture of hexaborides and boron using a laser-heated diamond anvil cell under high-pressure and high-temperature conditions. Contrary to the expectation that lighter lanthanide elements require higher pressure to crystallize RB12, in situ X-ray diffraction experiments reveal that cerium dodecaboride crystallizes at 26 GPa, which is significantly lower than that required to form the heavier praseodymium dodecaboride (35 GPa). In addition to the lower formation pressure, an anomalous volume reduction is also observed in CeB12, which can be explained by a valence fluctuation between Ce3+ and Ce4+ indicated by X-ray absorption near-edge structure measurements. A polyhedral coordination change from a truncated cube in RB6 to a truncated octahedron in RB12 and associated shortening of the R-B bond length result in an increase in bulk modulus and hardness.