The effect of hydrogen treatment on both GaAs pseudomorphic HEMT's and InP-based HEMT's, in order to simulate the hermetic seal environment in a Kovar package, is reported for the first time. Under the 270/spl deg/C, 4% H/sub 2/ in Ar atmosphere, significant changes in both types of HEMT's were observed within several minutes. While the drain current at a fixed gate bias and the pinchoff voltage of the GaAs PHEMT consistently decreased under the influence of the hydrogen gas, they were found to either increase or decrease with the InP HEMT. The change of device characteristics resulting from exposure to the hydrogen environment is not permanent; partial recovery of device characteristics was observed under either nitrogen or hydrogen at both elevated and room temperatures. The change in HEMT DC characteristics seems to be primarily resulted from the change in the gate built-in potential. Any device changes due to the Si-donor neutralization by atomic hydrogen, and therefore a reduction in channel carrier concentration, were found to be insignificant.<<ETX>>