The luminescence and quantum efficiency of the Eu 3− ion in CaO: Eu 3+ , Na − is reported. Center selective emission spectra provide information on the Eu 3+ centers in CaO, which partly consist of associates of the Eu 3+ ions with the available charge compensators, viz., calcium vacancies or Na + ions. The average quantum efficiency of the Eu 3+ luminescence in CaO under charge-transfer excitation ( q CT ) amounts to about 50%. The rocksalt lattice of CaO suppresses the radiotionless relaxation rates in the charge-transfer state to a large extent, but not completely. The effectively positive charge of the Eu 3+ ion in CaO (and other calcium compounds) seems to set an upper limit of about 50% to q CT in calcium compounds.