Reliability studies of InAlAs/InGaAs metamorphic high electron mobility transistors (MHEMTs) grown on GaAs substrates for high frequency/power applications are reported. The MHEMTs were stressed at a drain voltage of 3 V for 36 h, as well as undergoing a thermal storage test at 250 degrees C for 48 h. The drain current density of the MHEMTs at zero gate bias dropped about 12.5% after either the thermal storage experiment or dc stress. The gate current of the MHEMT devices with thermal storage was much higher than that of devices after dc stress. In the latter case, significant gate sinking was observed by transmission electron microscopy. The main degradation mechanism during thermal storage was the reaction of the Ohmic contact with the underlying semiconductor.