Long-time photooxygenation of b-pinene (I) with singlet oxygen leads to a mass product distribution. The yield of myrtenol (II) is a little improved by electron-transfer photooxygenation with dicyanoanthracene (DCA), but is almost double by the addition of biphenyl as cosensitizer. Competitive electron transfer of biphenyl to DCA over I has been proved to stem from higher HOMO of biphenyl by means of MNDO calculation Biphenyl radical cation gets one electron back from I to recycle biphenyl and form I radical cation, which combines with superoxide ion generated from electron transfer between ground oxygen and (DCA)-.

Key words: PHOTOCHEMICAL REACTION, CALCULATION, PHOTOOXIDATION, SENSITIZATION, PINENE, XENENE, MNDO METHOD, MYRTENOL