The complex formation between cobalt(II) and chloride ions in molten NH4NO3・2.61H2O, NH4NO3・CH3CONH2・1.61H2O and NH4NO3・2.61CH3CONH2 has been investigated. Absorption spectra of cobalt(II) chloride containing variable amounts of ammonium chloride were recorded at 45 to 60 °C. In the absence of chloride, the solutions show spectra typical for octahedral co-ordination of cobalt(II). Addition of chloride caused a shift of the absorption maximum toward lower energies and an increase of the molar absorption coefficient with increasing chloride concentration. The position of the absorption maximum and the intensity of the absorption indicate tetrahedral or severely distorted octahedral co-ordination. The stability constants for [Co(NO3)4]2−, [Co(NO3)2Cl2]2− and [CoCl4]2− complex formation in NH4NO3・2.61CH3CONH2 are reported.