Terpenoids with the general formula C15H24 and their oxy derivatives are an interesting class of natural products, which occur in many plant species. The structural formulas of the compounds studied are shown in Figures 1−3. Most of them are used as fragrance ingredients, while some (e.g., 9) are plant growth regulators. The current knowledge of the mechanism of olfaction and the structure of odorant receptors is limited, and the olfactophore models had been proposed as heuristic aids in computer design of new fragrances. Besides relevance for the mechanism of olfaction, the electronic structure of terpenoids makes an interesting case study of substituent effects. The reports of nonbonding interactions between several functional groups within the molecule, obtained by UV photoelectron spectroscopy (UPS), have been reviewed previously at considerable length. Here we are interested in the single functional group and how its (de)stabilization can be used as an indicator of substituent effects within the isomeric series. UV photoelectron spectroscopy (UPS) combined with MO calculations was shown to be a good method for studying the electronic structure of natural products.