From electron paramagnetic resonance (EPR) spectra of numerous semiquinone anion radicals we propose that the spin density distributions in benzo-, naphtho- and anthrasemiquinones are changed under the influence of various substituent in a comparable fashion, electron donating substituents having the opposite effect of withdrawing ones. These findings facilitate the work in obtaining correct assignments, further leading to safe identification of new radicals. Semiquinone anion radicals of para- and ortho-hydroquinones are demonstrated to exist at physiological pH in buffered solutions under continuous access to molecular oxygen. Reaction schemes are proposed for the formation of primary and tertiary radicals and hydroxylated products of ten quinols. From studies on crude plant extracts are three quinols, known as precursors in the biogenesis of chimaphilin, observed and identified in Chimaphila umbellata. Other studies demonstrate the power of using semiquinone radicals as chemotaxonomic markers.