The electrochemical etching of (110) n-Si surfaces in aqueous NH4F has been studied for the first time by in situ atomic resolution scanning tunneling microscopy. It was found that the degree of interfacial order was profoundly dependent upon applied potential. At potentials near or positive of the open-circuit potential, pit corrosion occurred that result ed in disordered surfaces. At potentials negative of the open-circuit potential, highly anisotropic layer-by-layer dissolution took place that rendered a rough surface atomically flat. The scanning tunneling microscopy images clearly showed an interfacial structure characterized by zigzag chains along the [110] direction. The distance between every other Si atom on the zigzag chain was measured to be 3.8 Angstrom, while the chain periodicity was determined to be 5.4 Angstrom. These values serve to establish that controlled negative-potential etching resulted in the formation of an ideal H-terminated (110) Si:H - (1 x 1) structure. Time-dependent scanning tunneling microscopy was employed to gain insight into the role of dihydride Si atoms in the anisotropic etching process. High-resolution images allowed the evaluation of absolute etching rates along the [110] direction through an exact numerical count of the Si atoms dissolved per unit time.