Chemical etching characteristics of GaAs(100) surfaces in aqueous HF solutions have been studied using spectroscopic ellipsometry (SE), atomic force microscopy (AFM), X-ray photoelectron spectroscopy (XPS), and wettability measurements. The SE data clearly indicate that the HF solutions cause the efficient removal of the GaAs native oxide upon immersing the samples in the solutions. When the native oxide is completely etch-removed, the resulting surface has a roughened overlayer about 1.3 nm thick, which is about five times as large as the AFM rms value (similar to0.2-0.3 nm). The discrepancy of this is due to the SE technique being sensitive: to both the surface microroughness and the adsorbed chemical species, but AFM is sensitive only to the surface microroughness. The XPS data reveal that etching in the concentrated (50 wt %) solution selectively removes gallium while leaving elemental arsenic on the GaAs surfaces. The XPS data also indicate the presence of fluorine on the HF treated surfaces. The as-degreased surface is hydrophilic, while thr W-treated surfaces are highly hydrophobic.