Two different microwave ECR sources have been examined for use in dry etching of GaAs, InP and related compounds. The first, a multipolar, tuned-cavity disk source (Wavemat) has been in operation for ∼4 years and provides stable, uniform discharges of BCl3/Ar, CH4/H2, SF6/Ar, HI/H2 and CCl2F2 for smooth, anisotropie pattern transfer into the III-V materials. Electron densities of -5 × 1011 cm-3 are achieved at 1 mtorr for microwave powers of >700W, as determined by microwave interferometry. Additional rf (13.56 MHz) biasing of the sample position is necessary for achieving practical etch rates. The second source is a Astex ECR high-profile geometry, with a drift-tube and secondary magnets near the sample position to improve the plasma confinement and etching uniformity. Similar electron densities are found as with the resonant cavity source, and optimization of the top and bottom magnet currents produces similar etch uniformities. Examples of the etching characteristics of III-V's as a function of microwave power, rf-induced dc bias and sample temperature are given.