The present work refers to the structural and luminescence changes in Ca1-xO:Bi (x = 0.5%) phosphor due to the reaction of the Ca1-xO: Bi (x = 0.5%) phosphor with atmospheric air. The X-ray diffraction (XRD) patterns of the as-annealed sample (at 1200 degrees C) showed the characteristic peaks of CaO, while high-intensity peaks of Ca(OH)(2) were also observed for the same samples that were kept in the air after annealing. Indicating that mixed phases of Ca1-xO:Bi (x = 0.5%) and Ca(OH)(2):Bi were present in the exposed samples. It was clear that the observed optical properties (excitation and emission) of the Bi3+ depended strongly on the reaction of the CaO with the environment due to the host lattice structure changes and therefore a big challenge for practical applications. (C) 2017 Elsevier B.V. All rights reserved.