The singlet-triplet energy splitting of phenylphosphinidene is predicted to be -21 kcal/mol at the Davidson corrected TC-CISD/6-31G* / /CISD/6-31G* level of theory. The ΔEs-t of phenylphosphinidene is the same as that of phenylnitrene, which also has a triplet ground state. The open-shell singlet and closed shell singlet states reverse their order when the group 15 elements is changed from N to P. Comparisons between NR and PR (R H, methyl and phenyl) show that the phenyl group strongly stabilizes the S0 state of phenylnitrene whereas there is no significant change in ΔEs-t for phenylphosphinidene. Methyl substitution leaves Δs-t unchanged for both nitrene and phosphinidene. © 1995.