The phosphor MgS:Ce,Ni is an efficient cathodoluminophor with green to yellow emission and very short decay time. The addition of nickel produces lower efficiency and correspondingly faster decay, the exact relation depending on the cerium concentration. The optimum composition for high efficiency (5–10 lm/W) and fast decay (0.25–0.35 &mgr;sec) was found to be MgS:Ce (0.001), Ni (0.0002). The primary green luminescence was shown to result from cerium doping rather than from some other impurity.