The degradation of polyethylene caused by vacuum ultraviolet radiation emitted from a synchrotron was investigated by in situ ultraviolet photoelectron spectroscopy (UPS). Excellent agreement is obtained between the UPS of pristine polymer and the density of states calculated by ab initio molecular orbital method for n-decane, five units of an ethylene monomer. The UPS of degraded polymer is analyzed by comparison with molecular orbital calculations of degraded polymer models. The analysis indicates the presence of conjugated double-bond and cross-linked structures in the degraded polymer. The degree of the pi conjugation generated upon the photodegradation is discussed.