Conformational Stability, 1H NMR o,o′-Bis(phenylethynyl)-triphenylphosphineoxide, coordinated to Pd(II) with the P=0-group, is isomerized in boiling xylene to the condensed phosphepin- and dihydrophosphocin oxides (4) and (6). The structure of 6 is determined by catalytic hydrogenation, 1H NMR and confirmed by an X-ray analysis, currently carried out. The hydrogenated products 7A and 7B are conformational isomers. The kinetic data of the ring inversion 7 A ⇄ 7B are determined.