The effects of electron bombardment in 10(-6) Torr CO or CO2 upon the surface composition of ZnS thin films was determined with Auger electron spectroscopy. The ZnS surface composition war; stable in 1x10(-9) Torr residual vacuum or for CO and CO2 exposures without electron bombardment. With electron bombardment and CO both present, C was deposited and S decreased, but no O accumulated on the surface. This was interpreted as an electron beam stimulated surface reaction in which CO was dissociated by the electron beam CO2 was reformed leaving C on the surface. For CO2, both C and O were increased on the surface while S was reduced. This was interpreted as a surface reaction involving dissociation of CO2 with reformation of COx leaving both C and O on the surface. The data suggest that the surface dissociation of CO2 did not occur when the sample temperature was increased from 30 degrees C to 150-200 degrees C. At 300 degrees C, S diffused from the ZnS substrate to the sur-face. The consequences of these reactions on ZnS cathodoluminescent phosphors are discussed.