Cobalt(II) complexes containing water and isocyanides (MeNC and PhNC) as the only ligands have been reinvestigated. The solid blue pentakis(phenyl isocyanide)cobalt(II) perchlorate, previously assumed to be an anhydrous compound and to contain a five-co-ordinate complex, is now shown to be the sesquihydrate and to contain the six-co-ordinate complex ion [Co(PhNC)5·H2O]2+. This blue solid can, however, be reversibly dehydrated to a yellow anhydrous compound, containing the five-co-ordinate square pyramidal complex ion [Co(PhNC)5]2+. Comparison of the reflection and absorption spectra shows that analogous six-co-ordinate complexes exist in solutions of the pentakis-(methyl and phenyl isocyanide)cobalt(II) compounds. Study of the Co(ClO4)2–MeNC system in aqueous solution shows the formation, at high MeNC concentrations, of a very unstable mauve coloured complex, probably [Co(MeNC)6]2+, and the absence of any complexes with less than five isocyanides per cobalt atom.