Both electrical and optical properties of AlGaAs and InGaP were changed by exposure to high density Ar and SF6 plasmas. Capacitance-voltage measurements, photoluminescence, and sheet resistance measurements were used to characterize the degradation of the materials properties as a function of microwave power, rf power, exposure time, and chamber pressure. The results showed the n-type InGaP suffered more damage than the p-type InGaP in Ar plasmas, while the reverse was true for AlGaAs. Annealing of the damaged semiconductors needed high temperature (for example, similar to 750 degrees C for InGaP), which is not suitable for typical III-V semiconductor processing because the high temperature may cause thermal degradation. Electron cyclotron resonance (ECR) SF6 plasmas produce more surface states than reactive ion etching (RIE) (i.e., rf power only). Both C-V measurements and photoluminescence data showed rf power (>50 W) and pressure (<2 mTorr) played important roles in introducing electrical and optical degradation of both AlGaAs and InGaP under ECR conditions, while there was little change as a function of rf power in the RIE case. The minimal overetching time of SF, plasmas is necessary to avoid severe damage in the materials during opening the windows for the active semiconductors from photoresist, dielectric, or metal layers.