Spectroscopic investigations have been performed for the oxidized forms of two quadruple-decker phthalocyanine complexes in order to clarify the electronic structures of multiply stacked pi-systems. Up to three-electron-oxidized species were isolated by using phenoxathiin hexachloroantimonate as the oxidant. As the oxidations proceed, the Q:bands in the visible region shift bathochromically along with the clear isosbestic points. The one- and three-electron-oxidized species exhibited typical pi-radical signals in the ESR spectra, while the neutral and two-electron oxidized species gave no indication of the presence of pi-radicals. The electronic transitions observed for the oxidized species reach even into the so-called fingerprint region in IR spectroscopy (similar to 1000 cm(-1)). With the aid of theoretical calculations, these bands can be assigned to the pi-pi* transitions. Our result; provide new insights into pi-electronic systems having exceptionally small MO energy gaps.