The HeI photoelectron spectra of 9 isomeric [b,d]-annelated dithienopyridines, e.g., I, are reported and discussed including those of some closely related compds. such as phenanthridine and dithieno[3,4-a:3',4'-c]benzene and its tetrahydro deriv. The ionization potentials are assigned to MOs using the results of MNDO, AM1 and PM3 calcns. From the spectra as well as from the theor. results, the isomers can be divided into two groups, depending on the mode of annelation. The crystal and mol. structures of one member of each group are detd. by x-ray diffraction. The mode of annelation is also reflected in the bond lengths of the pyridine and thiophene rings.