The values of the Lewis acidity parameter (acceptor number) for acceptor compounds (ANν) and twenty binary solvent mixtures (ANm) were estimated using the solvatochromic dicyanobis(1,10-phenanthroline)iron(II) complex. The acceptor number values for the acceptor compounds (ANν) are linearly correlated with the Taft constant (σTaft) and the formation constant values. On the other side, ANm values for binary solvent mixtures over the whole composition range of the mixture are correlated with the other solvent parameters and equilibrium data. These correlations suggest the importance of the present ANm values to correlate the reaction rates, and may be used as an aid in the interpretation of equilibrium studies in solvent mixtures. Equilibrium constants, for the stepwise interaction of one or two acceptor ligand molecules with Fe(phen)2(CN)2 in DMF solution at 25 °C to form adducts, Fe(phen)2(CN)2(A)n, have been evaluated using a spectrophotometric technique. The equilibrium constants obtained were correlated with the values of the dissociation constant (pKa), acceptor number, and σTaft of the acceptor ligands.