The cathodoluminescence (CL) brightness from thin film Ta2Zn3O8 has been studied as a function of vacuum and electron dose. The thin films were prepared by sputtered deposition followed by rapid thermal annealing at 1100 degreesC for 1 min. Under bombardment by 2 keV electrons, the films produce a blue luminescence with a dominant wavelength of 386 nm. These films were exposed to residual vacuum gas dominated by H-2 and H2O at pressures ranging from 10(-8) to 10(-6) Torr with <10% loss in CL brightness for an electron dose of 23 C/cm(2). However, when hydrocarbons from colloidal graphite "paint'' raised the base pressure of the vacuum from 2 X 10(-7) to 9 X 10(-7) Torr, the CL brightness was degraded to 5% of its original value after 4 h at 2.7 X 10(-4) A/cm(2) (corresponding to a dose of 3.9 C/cm(2)). Degradation stopped when the electron beam was off or could not strike the surface. Auger analysis showed a 1000-Angstrom -thick layer of carbon from electron beam cracked hydrocarbons, The effect of electron beam cracked carbon was concluded to be absorption of luminescent photons. The hydrocarbon contaminants were removed by long term baking of the vacuum system.