Phosphorescence and optically detected magnetic-resonance (ODMR) expts. are reported for the lowest excited triplet state of 4-PhCOC6H4Ph, (4-PhC6H4)2CO (I), Ph2CO, 4-PhCOC6H4C6H4COPh-4, and 1-benzoyl- and 1,5-dibenzoylnaphtalene. These compds. were studied in single-crystal form and have a lowest pp* triplet state. The contribution of the magnetic dipole-dipole interactions to the zero-field splitting is detd. and compared with data for benzaldehydes and acetophenones. An isotope effect was seen in the ODMR spectrum of the triplet state of I 4,4'-dichloro deriv., which appeared to be present in I.