In situ real-time and electrochemical attenuated total reflection Fourier transform infrared (ATR-FTIR) spectroscopy has been employed to study the dependence of hydride structures of an Si(111) surface in diluted NH4F solution on etching time and applied potential. The in situ real-time ATR-FTIR result shows that the Si(111) surface remains H terminated in diluted NH4F solution, and the hydride on Si(111) has a strong interaction with water molecules under cathodic potential control. Potential dependence of the in situ electrochemical ATR-FTIR spectra reveals that Si(111) surface is free of oxide at the open circuit potential (OCP) and SiHO3 is formed only at potentials positive of the OCP. Infrared roughness factors, defined as the ratio of monohydride at steps or dihydride over the monohydride on the (111) plane, suggest that monohydride chain steps are the dominating defects on the Si(111) surface in NH4F at cathodic electrode potential. Horizontal (D') and vertical (D) dihydrides are predominant on the Si(111) surface leading to roughened morphology via oxidation at anodic potential.