Photooxidation reactions of triphenylphosphine (Ph 3 P) monomers isolated in matrices of solid oxygen at 10K were characterized by means of infrared spectroscopy. Upon UV (λ>280nm) irradiation of O 2 matrices, ca. 90% of Ph 3 P was converted to triphenylphosphine oxide (Ph 3 PO), with concomitant formation of ozone. In the competing photoreaction, ca. 10% of Ph 3 P was converted to diphenyl-phenyl-phosphinate, Ph 2 (Ph−O−)PO. The interpretation was assisted by theoretical [DFT(B3LYP)/6-31G(d,p)] calculations of vibrational spectra, as well as by comparison with the experimental vibrational data from separate experiments in which monomeric Ph 3 P and Ph 3 PO were isolated in argon and oxygen matrices at 10K.