Studies on Ln[Co(CN)(6)]. nH(2)O (Ln = lanthanoid ions; n = 5, 4) by means of thermal analysis, Raman spectroscopy, and X-ray crystallography were carried out, in order to establish the boundary structures in the series. From the thermal analyses, it was confirmed that the complexes include Ln’[Co(CN)(6)]. 5H(2)O (Ln’ = La to Nd) or Ln "-[Co(CN)(6)]. 4H(2)O (Ln = Sm to Lu). Raman spectra of the complexes suggested a different classification. The complexes having five H2O molecules displayed two single bands associated with v(C-N) at around 2170 cm(-1). The complexes having four H2O molecules showed two distinct sets of bands of v(C-N) : one was a singlet, and the other was split. Nevertheless, the complex with Nd, which has five H2O molecules, exhibited single and split bands. This implies that the symmetry around Nd is lower than that of other complexes having five H2O molecules. According to the X-ray crystal analysis, the Pr complex is Pr[Co(CN)(6)]. 5H(2)O, hexagonal, P6(3)/m, with a = 7.473-(1) Angstrom, c = 14.212(1) Angstrom, and Z = 2. On the other hand, the Nd complex is Nd[Co(CN)(6)]. 5H(2)O, orthorhombic, C222(1), with a = 7.458(4) Angstrom, b = 12.918(3) Angstrom, c = 14.172(2) Angstrom, and Z = 4. Although the Nd complex has five H2O molecules, the crystals are orthorhombic and belong to the space group C222(1). Therefore, the structure of Nd[Co(CN)(6)]. 5H(2)O is regarded as the boundary structure : one of the coordinated water molecules is disordered, although the structure is essentially the same as that of Pr[Co(CN)(6)]. 5H(2)O. As Pr in Pr[Co(CN)(6)]. 5H(2)O. changes into Nd, the symmetry around the metal atom is lowered and thus the bands associated with v(CN) in Nd[Co(CN)(6)]. 5H(2)O and Sm[Co(CN)(6)]. 4H(2)O outnumber those of Pr[Co(CN)(6)]. 5H(2)O. The 5H(2)O complex with Nd loses one water molecule by thermal dissociation and changes into the more stable 4H(2)O complex, whose crystals are orthorhombic and belong to the space group Cmcm. Pr[Co(CN)(6)]. 5H(2)O also changes into the 4H(2)O complex, orthorhombic and Cmcm, when it dehydrates.