Photoelectron spectra of indene, naphthalene, azulene, biphenyl, quinoline and isoquinoline are reported, and some inner ionization potentials of naphthalene, azulene, biphenyl and biphenylene measured by an electron impact method are also given. A close correlation is found between the inner ionization potentials of the hydrocarbons and π-electron orbital energies given by the zerooverlap Hückel approximation.