Photoluminescence and low-voltage cathodoluminescence characteristics of ZnGa2O4 phosphor doped with monovalent ions has been studied. Monovalent ions such as Na+ and Ag+ are incorporated into ZnGa2O4 lattices in order to increase the concentration of oxygen vacancies in the spinel lattice. By doping low concentrations of monovalent ions (Na+, Ag+) into ZnGa2O4, the self-activated blue luminescence originated from oxygen vacancies is enhanced. Also, the blue luminescence intensity is enhanced more along with a good color purity by annealing ZnGa2O4:Na+ in a reducing atmosphere, which is due to increasing the concentration of oxygen vacancies even more. The luminescence band at the UV region (lambda(max) = 360 nm) does not become the major luminescence band by introducing Na+ ion into the ZnGa2O4 lattice, while the UV luminescence band becomes the major one by annealing the undoped ZnGa2O4 in a reducing atmosphere. (C) 2002 Elsevier Science Ltd. All rights reserved.