High power, high efficiency GaAs IMPATT diodes in pulsed mode operation are characterized in this study. Experimental results are presented for the R.F. power and frequency drift versus current and temperature characteristics when the diodes are pulsed in a very efficient, flexible microwave resonator. A novel analytical GaAs IMPATT diode model is described and used to verify the frequency drift versus time results obtained experimentally and to predict that a current frequency tuning phenomenon can be "used to minimize the frequency drift of the IMPATT diodes when operated in pulsed mode. A new pulsed constant current bias supply suitable for the high power GaAs IMPATT diodes is also described and used throughout this study.