We present He I photoelectron spectra (UPS) of several isomeric monocyclic trienes and dienes. The spectra were interpreted by semiempirical MO calculations and comparison with spectra of related compounds. The results suggest that in 1,5,9-cyclododecatriene the homoconjugative π,π-interactions are sensitive to E/Z molecular topology while in 1,9-cyclohexadecadienes they are not. We have analyzed both π-ionization energies and σ-onset energies in order to unravel relative importance of through-space and through-bond interactions. We propose a new trans rule for monocyclic olefins.