AbstractTwo centers with tetragonal symmetry and principal axes along 〈100〉 are observed by ESR in single crystals of CaO. The ESR signal intensity of both the centers is affected by visible light illumination. One center (g∥ = 2.0048,g⟂ = 2.0007) occurs in highly colored crystals and may be associated with carbon impurities introduced during crystal growth. The other (g∥ = 1.99905,g⟂ = 1.99956) occurs in “clear” crystals after 1 MeV electron irradiation and has the characteristics of an anion–cation vacancy pair center containing one electron (i.e. a P−or F c+center). Theg‐values do not agree with those found for the same center in polycrystalline CaO but are consistent with a simple model