The vertical and adiabatic singlet-triplet energy splittings (Delta E-ST) of phenyinitrene were computed by a variety of multireference configuration interaction and perturbation theory methods employing basis sets of up to quadruple-xi quality and extrapolation to the complete basis set limit. The vertical and adiabatic energy gaps are 18.9 and 15.9 kcal mol(-1), respectively, the latter in reasonable agreement with the revised experimental value of 15.1 +/- 0.2 kcal mol(-1). The energy difference between both states at the geometry of the (a) over tilde (1)A(2) singlet state was also considered and amounts to 13.8 kcal mol(-1). In obtaining accurate state energy splittings, basis set completeness turns out to be a more important issue than the level of dynamical electron correlation treatment. Density functional theory that is frequently employed to investigate phenylnitrenes and their rearrangements yields varying results and, depending on the functional, gives adiabatic energy differences between 9 and 16 kcal mol(-1). The (b) over tilde (1)A(1) state has a similar geometry as the ground state of I and is 31 kcal mol(-1) higher in energy. According to best estimates, the next higher singlet states, (c) over tilde (1)A(1) and (d) over tilde B-1(1), are 57 and 72 kcal mol(-1) above the ground state. In the triplet manifold, vertical excitation energies to the A 1131 and (B) over tilde (3)A(2) states are 71 and 77 kcal mol(-1), respectively.