The photosensitized oxidation of the exo, endo transoid diene, neoabietic acid, was found to yield a crystalline diperoxide; analytical and chemical evidence indicate the structure. Pimaric, isoimaric and dehydroabietic acids were found to be unreactive. A transition state is proposed which appears to accommodate many of the known facts regarding the photosensitized oxidation of olefins. Data are cited to suggest the configurations for all asymmetric centers in palustric acid and for the C-13-hydrogen atom in levopimaric acid. The conclusion is drawn that all seven of the major pine gum resin acids contain the same absolute configurations in the asymmetric backbone chain (positions 1, 11, 12 and 13) with the exception of the C-13-hydrogen atom in isopimaric acid.