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This work applies theoretical and computational methods to investigate the relationship between phosphorescence lifetime and the electronic character of the lowest triplet state of aromatic carbonyls. A formal analysis of the spin-perturbed wave functions shows that phosphorescence is due to a direct spin–orbit coupling mechanism modulated by permanent dipoles when the T1 minimum is 3nπ*. If the minimum is a totally symmetric 3ππ*, phosphorescence is due to an indirect spin–orbit coupling mechanism involving transition dipole moments with other excited states. The magnitude difference between permanent and transition dipoles leads to a much faster 3nπ* phosphoresce than 3ππ*. These predictions were verified with phosphorescence lifetime simulations of benzaldehyde and its three derivatives in the gas phase employing a vertical approximation and the nuclear ensemble approaches. Both predict 3nπ* emission within a few tens of milliseconds. While the vertical approach indicates a 3ππ* emission within a few seconds, vibronic corrections bring this value down to about 200 ms. [ABSTRACT FROM AUTHOR]