Correlation with acceptor numbers was made for the solvatochromic behaviour ofbis-(2,2′-bipyridyl)-bis-cyanoiron(II) in binary aqueous mixtures of acetonitrile andDMF. The relationship between isosolvation point and donor number of the organic cosolvent was proved to be linear. Preferential solvation occurs by water in the case of the binary acetonitrile mixtures and byDMF in the case of the binaryDMF, aqueous mixtures.