Abstract (1) In order to verify the postulate that the third band is due to a pair or pairs of negative radicals co-ordinated in trans-positions to each other, extinction coefficients of [Co(NO2)6]Na3, [Co(NH3)4Cl NO2]Cl, [Co(NH3)4(NO2)2(6)(1)]Cl, [Co(NH3)2(NO2)4]NH4, etc. were measured. (2) Aqueous solutions of [Co(NO2)6]Na3 and [Co(NH3)4Cl NO2]C1 were proved to have the third bands as will be predicted from the postulate. (3) Regularities among the frequencies of the first, the second, and the third band were pointed out, viz., for the cobalt complex salts, which show the third band, ν3–ν2 and ν2–ν1 are 33×1013 and 18×1013, whereas for the pentammine and the tetrammine cobaltic salts, ν2–ν1 is 27×1013 and 24∼26×1013 respectively.