Infrared spectra (300—4000 cm -1 ) have been measured on the octacyanide complexes K 4 Mo (CN) 8 ·2H 2 O and K 4 W (CN) 8 ·2 H 2 O in aqueous solution, in the solid dihydrates, and in the solid anhydrous compounds. Comparison with relevent Raman data, particularly in the 2100 cm -1 region, provides conclusive evidence for the effective molecular symmetry of the M (CN) 8 4- group in the different surroundings. In aqueous solution, the ions [Mo (CN) 8 ] 4- and [W (CN) 8 ] 4- assume the symmetry of an Archimedean antiprism, D 4d . In the dihydrate crystals, the groups Mo (CN) 8 4- and W (CN) 8 4- approach the full D 2d symmetry of a dodecahedron with triangular faces, possibly slightly distorted by packing effects. In the anhydrous compounds, the symmetry of these groups is considerably lower, very likely C 8 . Reasonable assignments for all normal frequencies in the 2100 cm -1 region are proposed, both in D 4d and D 2d symmetry.