Absolute configurations of C-2 and C-3 in “Acid III” (IIa), a degradation product of piericidin A diacetate, was determined as R and S, respectively. Consequently both C-9 and C-l0 in piericidin A were confirmed to have S configurations. On the basis of UV and NMR specta, configurations of the double bonds in piericidin A were assigned as shown in Ia.