The photoelectron (PE.) spectra of 1,2,3-methenoindane (2), 1,2,3-metheno-2,3-dihydro-1H-cyclopenta [b]naphthalene (3) and 1,2,3-metheno-2,3-dihydro-1H-phenalene (4) are investigated. The PE. spectrum of 1,3-methano-2,3-dihydrophenalene (7) is reported and compared with that of 4. The experimental results are analyzed in terms of empirical correlation diagrams and the results of semiempirical MINDO/3 and CNDO/S calculations. The analysis indicates that the strong impact of the bicyclobutylene group on the spectroscopic properties of the aromatic π-systems in 2, 3 and 4 is due to hyperconjugative interactions involving bicyclobutane Walsh orbitals.