Cathodoluminescence (CL) properties, crystal structures, and bandgap energies (E-g) of Zn1-xMgxS:Cu,Al powder phosphors with a wide range of Mg content were investigated because ZnS phosphors are one of the most important types of phosphors for cathode-ray tubes and field emission displays. The investigation showed that irradiation of the phosphors by an electron beam (5 mu A/cm(2) at 27.5 kV) shifted the CL colors from green to blue with increasing Mg content. In the CL spectra, the green-copper band shifted to blue and a new blue band appeared at 2.86 eV. Moreover, an increase in E-g caused a blueshift of the green-copper band when the Mg content was less than 10 mol %. The E-g was calculated by an ab initio calculation using the Cambridge Serial Total Energy Package (Accelrys, Inc.) software package. This calculation showed that lattice expansion decreases the E-g of Zn1-xMgxS, and the calculated E-g (including the effect of the lattice expansion) agrees well with the measured E-g.