In this letter, we have investigated hydrogen degradation of InP HEMT's with Ti/Pt/Au gates. We have found that V/sub T/ shifts negative after exposure to hydrogen, and exhibits an L/sub G/ and orientation dependence. We postulate that /spl Delta/V/sub T/ is at least in part due to the piezoelectric effect. Hydrogen exposure leads to the formation of TiH/sub x/, producing compressive stress in the gate. This stress induces a piezoelectric charge distribution in the semiconductor that shifts the threshold voltage. We have independently confirmed TiH/sub x/ formation under our experimental conditions through Auger measurements. Separate radius-of-curvature measurements have also independently confirmed that Ti/Pt films become compressively stressed relative to their initial state after H/sub 2/ exposure.