ZnS:Cu,Au,Al thin film phosphors grown on Si(1 0 0) were subjected to an electron beam irradiation with beam energies ranging from 1 to 5 keV and current densities between 16 and 89 mA at O-2 and CO2 partial pressure of 5 x 10(-7) Torr. Auger electron spectroscopy (AES) and the cathodoluminescence (CL), both excited by the same primary electron beam were utilised to monitor the changes in the surface composition and the luminous efficiency of the films during electron bombardment, respectively. The degradation mechanism for these reactive gases in ambient was manifested by the depletion of S and the formation of a non-luminescent ZnO layer on the surface of the phosphor. The degradation was more severe under O-2 ambient compared to the same partial pressure of CO2 during electron beam bombardment, which is attributed to the free energy of formation of ZnO from ZnS when these respective gases are used. The degradation rate also depends on the energy of the electron beam, decreasing with increasing beam energy. This was interpreted according to the ionisation energy cross-section profile. The CL brightness increased exponentially with the increasing beam energy as more free carriers that will subsequently recombine yielding CL, are excited at higher beam energies.