The structures of the ground state phenoxy radical and phenylnitrene were calculated by the UNO-CAS method and compared with CAS-SCF results. The two methods produce almost identical structures which are very sensitive to polarization functions in the basis sets. On going from the 3-21 G to the 6-31 G* basis set, the CO and CN distances decrease significantly. Further enlarging the basis set from the 6-31 G* to the 6-311 G(2d, p) has minor effect on the calculated structures. These results indicate that for the correct description of the title compounds, a basis set of at least double zeta plus polarization quality is needed, and one should be cautious about predictions derived from single point calculations on structures obtained with the 3-21 G basis set.