Addition of oxygen in the phosphor has been found to nearly double the efficiency and shift the emission spectrum to a deeper blue. To understand the effects that oxygen has on the emission spectrum, we used a self‐consistent‐field configuration interaction (SCF/CI) model to calculate the electronic spectrum of oxygen‐doped SrS:Ce. By replacing the appropriate number of sulfur ligands with oxygen ligands, the SCF/CI model predicted a blue shift for two different cerium‐oxygen bond distances that is in very good agreement with the blue shift observed for . An analysis of atomic orbital electron populations indicates that changes in covalency of the Ce 5d orbitals upon incorporation of smaller ions causes the observed blue shift. Calculated oscillator strengths indicate that the observed increase in electroluminescent efficiency caused by addition of oxygen is not a result of changes in transition intensities. © 2000 The Electrochemical Society. All rights reserved.