In this letter, we present optical mixing in epitaxial lift-off (ELO) pseudomorphic HEMT's (PHEMT's) at difference frequencies in the microwave regime up to 22 GHz. The 3 mu m gate length AlGaAs-InGaAs PHEMT's were lifted off their host GaAs substrates and subsequently attached to quartz slides. It was observed that the ELO devices consistently resulted in stronger signals (similar to 7 dB) than the non-ELO devices under frontside and backside illumination, This is attributed to improved optical coupling efficiency, a decrease in substrate leakage, and an illumination-induced back gating effect for the ELO films.