Chemically treated Si(100) surfaces in an aqueous NaOH solution at 20 degrees C have been studied using spectroscopic ellipsometry (SE) and ex situ atomic force microscopy (AFM). The SE data indicate that a surface native oxide can be etched very slowly by NaOH etching. When the native silicon oxide is partly etch-removed, the resulting surface is very rough. This occurs because the solution does nor etch surface native oxide rapidly, but attacks bulk silicon vigorously. The AFM image confirms a roughened surface of similar to 5.5 nm rms. Just after the native oxide is etched away completely, the SE data yield the spectrum of a nearly flat surface, as also confirmed by the AFM observation.