The He(I) photoelectron spectra of thioindigo, selenoindigo, bi(4,4-dimethyltetrahydropyrrole-3-one-2-ylidene), bi(4,4-dimethyltetrahydrothiophene-3-one-2-ylidene), octahydroindigo, 4,4'-dibutyl-5,5'-dimethylpyrrolindigo, and thiophenindigo have been obtained by evapg. the compds. at temps. up to about 350 DegC. The ionization potentials (IPs) are compared with those of the parent compd. indigo and are related to orbital energies or electronic states of the resp. radical cations with the aid of semi-empirical SCF MO calcns. A satisfactory interpretation of the spectra is achieved with the Outer Valence Green's function method OVGF in combination with PM3 results. The first three IPs of all indigoid mols. in this study have the same origin, i.e. they relate to similar MOs. Because of the close relationship of the electronic structures of indigoid mols., the IPs of the unknown unsubstituted pyrrolindigo could be estd.