We have observed the optically-detected ESR transitions of the steroidal enone testosterone acetate at zero field and found the zero-field splitting (ZFS) parameters to be | D| = 0.28 cm − and | E| = 0.01 cm −. Approximate theoretical calculations suggest that there are at least three important contributions to the ZFS; spin-spin dipolar interaction, spin-orbit coupling of the emitting 3(π,π *) state with the 3(n,π *) state, and spin-orbit coupling of the 3(π,π *) state with the 1(n,π *) state. Thus the calculated ZFS parameters of D = −0.46 cm − and E = −0.014 cm − represent a delicate balance of these three effects. Because of the large spin-orbit coupling effects the ZFS parameters of enone 3(π,π *) and 3(n,π *) states, and perhaps ketones in general, depend strongly on the separation between (n,π) and (π,π) states. This is a complicating feature which may make it difficult to use ZFS parameters to assign the orbital character of ketone triplet states.