Simple measurements of the decay time of cathodoluminescence are described. Cathodoluminescence is used in many devices, including computer monitors, oscilloscopes, radar displays and television tubes. The experimental setup is simple and easy to build. Two oscilloscopes, a function generator, and a fast photodiode are needed for the experiments. The decay time determined using square wave and sine wave excitation of the phosphor appeared to be less than 10[superscript -4] s. (Contains 5 figures.)