Abstract.  Rare gas halide lasers are of interest for laser fusion facilities. The excited upper state of the lasing species is produced by plasmakinetic collisional processes which result from energetic electron beams, which provide the energy for the laser. Electron-beam pumped KrF and XeF lasers have been scaled up to large aperture sizes, and pulse energies greater than 6 kilojoules have been achieved. In some laser experiments, the laser output power fell unexpectedly before the electron beam pump pulse terminated. In addition, the laser efficiency did not increase when the gas temperature was increased, as expected from small-scale experiments. A computer model of XeF kinetics was used to investigate these questions. Two formation reactions involving Penning ionization and charge transfer were found to be important to understanding this behavior.