The logarithms of equilibrium constants for the charge-transfer interaction between iodine and a number of aza-aromatics have been found to be approximately proportional to the pK values of the respective aza-compound. The equilibrium constants for 2:6-lutidine and 2:4:6-collidine have been found to be much less than the theoretical values due to steric effect of the methyl groups at 2 and 6 positions. The steric effect has also been found to be responsible for complete suppression of complex formation in 7:8-benzoquinoline + I 2 system.