Electron beam stimulated surface chemical reactions (ESSCRs) have been shown to lead to degradation of cathodoluminescent (CL) phosphors. We have studied in detail the effects of various gaseous partial pressures on the processes leading to degradation. We have demonstrated that either high partial pressures of H₂ or H₂O will cause degradation of ZnS P22 phosphors, but the ESSCR is very different. Degradation is much more severe and occurs at lower coulombic loadings when partial pressures of » 10^-8 Torr of H₂O are present. This partial pressure was controlled over a wide range by back filling the vacuum chamber from a stainless steel vessel containing H₂O isolated from the vacuum by a leak valve. Hydrogen partial pressures of » 10^-8 Torr were sufficient to result in degradation by an ESSCR in which H₂S was speculated to be formed. The effects of CO, CO₂ and C_xH_y will be discussed. The implications of ESSCRs for FEDs will be discussed.