The influence of solvent and added solute molecules on the nature of the species observed in solutions of the tetrasodium salt of cobalt(II) tetrasulphophthalocyanine has been studied by electronic absorption spectrophotometry. Water is the only solvent which promotes formation of the known dimeric species. The equilibrium between monomer (M) and dimer(D) obeys the relationship K=[D]/[M]2·[H2O]n where n 12 in both water–methanol and water–ethanol mixtures. It is suggested that the water molecules play a specific role in binding together the two monomer units through the formation of hydrogen bonds.