Crystalline cobalticyanides LnCo III(CN) 6· nH 2O with Ln = La,…, Lu, Y have been synthesized by a double-infusion technique. In analogy to the Cr and Fe compounds, the large rare-earth ions form a hexagonal modification while the smaller ions lead to the orthorhombic structure with 4H 2O. Experiments show that no magnetic ordering occurs down to 1°K. The Stark splitting of the J ground state due to the crystalline field is analyzed for the Ce and Sm compounds.