Electron paramagnetic resonance (EPR) and optical spectral studies have been carried out on the pentacyanocobaltate(II) ion in ethylene glycol-water solutions, and in electron-irradiated powder samples of K3Co(CN)6. Both EPR and optical spectra observed for the ion in the powder samples are essentially similar to those obtained from the ethylene glycol-water solutions. There was no observable linewidth variation with nuclear spin states MI, no dependence on the different solvent media used, and no dependence on the concentration ratio Co2+/CN-, in both X-band and K-band spectra. The results indicate that the structure of Co(CN)53- in solutions and in polycrystalline media is a slightly distorted square pyramid with no solvent bound in the sixth coordination site. The two strong hyperfine components with exceptionally large spacings occurring at the low-field end of the X-band spectrum are identified as the so-called angular anomalies. An effective method of simulation of the first-derivative EPR spectra of polycrystalline samples has also been developed.