GaAs, a compound semiconductor commonly used in optoelectronic devices, is often subjected to wet-etching techniques during microelectronic device manufacture. In this work we investigated the wet etching of GaAs by H2O2-NH4OH-H2O solutions using a batch stirred-tank reactor and determined the intrinsic kinetics of the dissolution reaction. Increasing the NH4OH content produced a constant rate above a minimum concentration. The reaction rate was found, in the presence of excess NH4OH, to fit a rate equation r = k[H2O2](0.75) at 15, 25, and 40 degrees C with an activation energy of 33.7 kJ/mol. Using NaOH instead of NH4OH resulted in greatly reduced reaction rates, and it was concluded that the presence of the ammonium ion increases the rate by forming soluble compounds with oxidized species of Ga and As. Analysis of the surface by X-ray photon spectroscopy confirmed that samples etched in solutions containing NH4OH had considerably less oxide content on the surface than that etched in H2O2-H2O only. NH4OH does not directly react with GaAs but incorporates a second step into the overall etch reaction, facilitating the oxidation by H2O2 and the formation of soluble compounds.