The first example of triplet-state phosphoryl diradicals is reported. Photoinduced dissociative electron capture of two diastereoisomeric m-phenylenebis(phosphinic chloride)s in a glassy toluene matrix at 130 K using an electron-rich olefin affords the corresponding phosphoryl monoradicals and triplet-state diradicals in a 5:2 ratio. The triplet-state ESR spectrum gives zero-field parameters of D/hc = 0.0120 cm(-1) and E/hc 0.0015 cm(-1) Variable temperature ESR experiments reveal Curie behavior for the Delta m(s), = 2 ESR signals between 3.8 and 100 K, consistent with a low-energy triplet state that either corresponds to the ground state or is degenerate with a singlet state. For the corresponding p-phenylene diastereoisomers, no triplet-state diradicals can be detected under identical experimental conditions and the ESR spectra only reveal the primary phosphoryl monoradicals and their protonated forms, as secondary species.