He I photoelectron spectra (PE) of several β,γ-unsaturated polycyclic ketones, their parent ketones, and alkenes have been measured. The assignment of spectra is based on semiempirical MO calculations (PM3 method) and empirical arguments. Endo-derivatives showed larger HOMO/SHOMO band separation than exo-cyclic unsaturated ketones. The results of MO calculations are in good agreement with this general trend. Strikingly different photochemical transformations and π* ⟸ n electronic transition intensities, exhibited by the two groups of compounds, can be rationalized within the framework of oxygen lone pair ↔ π bond interactions.