The UV photoelectron spectra of two tricyclic heteroarom. compds., naphtho[2,3-b]thiophene (I) and naphtho[2,3-c]thiophene (II), that are p-isoelectronic with anthracene (III), have been recorded and analyzed making use of semi-empirical AM1 and PM3, as well as d. functional theory (DFT) B3LYP calcns. In I and II, one peripheral benzene ring of III is substituted by a thiophene ring that is either [b]- or [c]-annellated. In II and III, only small shifts are found for most of the ionization potentials of p electrons. Since the ionization energies of all occupied p MOs of I-III could be assigned, a direct comparison of their p electron energy is possible. Compared with III, the p-electron system of I is stabilized by 0.6 eV, while that of II is destabilized by 0.2 eV. [B]-Annellation of the thiophene ring is thus favorable while [c]-annelation is unfavorable.