The chemical composition of GaAs(100) surfaces after HCl/H2O wet etching sequences has been examined with x‐ray photoelectron spectroscopy (XPS). XPS measurements show that oxides and chlorides are removed from the GaAs surface by the HCl solution etching in 10–20 min and subsequent H2O rinse in several seconds. The surface of the GaAs substrate thus prepared has been examined with atomic force microscopy. The surface flatness is so improved by the HCl/H2O treatment that the surface undulation remains within a ±1 monolayer fluctuation over a 1×1 μm surface area.