A detailed measurement of the speci5c heat of the B 12 icosahedral compound TbB 41 Si 1.2 was made. TbB 41 Si 1.2 crystals were grown by the 6oating zone method. Magnetic susceptibility measurements indicated that an antiferromagnetic-like transition occurs at 18 K in these crystals. TbB 50 and isostructural TbB 41 Si 1.2 are the 5rst higher borides in which a magnetic transition was observed to occur. A hump in the speci5c heat is observed below 20 K, supporting the conclusion that a magnetic transition occurs in this system. An interesting magnetic 5eld dependence was observed. Near the metamagnetic critical 5eld of 31 kG, determined from magnetization measurements, the hump shifts to lower temperatures approaching 0 K. At 40 kG no hump is apparent. However, on further increasing the magnetic 5eld to 55 and 70 kG, a new hump in the speci5c heat appears, indicating the possibility of the existence of an additional transition at high 5elds. Magnetization measurements of REB 50 (RE โซุโฌ Tb, Dy, Ho, Er) up to 55 kG showed that the compounds saturated universally at around only half the saturation magnetization value of free rare-earth ions, which also indicated the likelihood of a further transition. The structure of the REB 50type compounds has a one-dimensional character with the rareearth atoms forming an alternating bond length chain along the c-axis. These results are quite interesting in the context with the recent high activity in Haldane gap related compounds in which fundamental quantum phenomena are observed. 2000 Academic Press