Screened samples of two different commercial phosphors were aged at 1.5 keV under nominally identical conditions in five different vacuum systems. After aging to doses of similar to 5 C/cm(2) the changes in cathodoluminescence (CL) spectral output and photoluminescence properties were determined, and x-ray photoemission spectroscopy (XPS) and Auger analyses were performed. We observed that while the CL degradation was similar for all systems at low electron doses, the aging rates diverged at high doses, suggesting an increasing importance of the vacuum environment. Post-aged CL spectra showed distinct redshifts in all cases, and the most heavily degraded samples displayed XPS-determined increases in surface carbon levels. The data suggest an extrinsic CL aging mechanism; a model involving the conversion of surface hydrocarbons to a metallic graphite phase agrees with the data.