Mulliken’s theory for donor-acceptor interactions is usedto rationalize the effects of aryl substituents on the intensities of the metal-to-ligand charge-transfer absorptions that are characteristic ofCu (I) complexes with phenanthroline ligands. Depending on the positions of the aryl substituents, the absorption intensity is enhanced or diminished with respect to the unsubstituted complex. The analysis verifies that the transition dipole length is approximately given by the separation between the charge centroids of the donor and acceptor orbitals. In the absence of steric complications, this means that the aryl substituent effects can be traced to readily envisioned resonance interactions. When conjugation with the system of the substituents extends the acceptor orbital out away from the metal center, the transition dipole length is increased and the intensity is enhanced. Conversely, a decreased transition dipole length results when the substituent spreads the acceptor orbital in a direction opposite to that of the optically induced charge transfer.