We have investigated the electrochemistry of Si(100) electrode surfaces under the negative bias condition in hydrofluoric acid (HF) solution using infrared absorption spectroscopy (IRAS) in the multiple internal reflection geometry (MIR-IRAS). We observe that immediately after the potential of −0.20 V is applied to a Si(100) electrode, dihydride (SiH 2) and trihydride (SiH 3) species increased their surface densities, while monohydride (SiH) decreases its density. When further applying a negative potential, no spectral changes were observed. We propose that the constrained bonds of surface hydride species are attacked by the hydrogen ion (H +) or the hydronium ion (H 3O +) to generate more SiH bonds in the vicinity of the surface, leading to an increase in the surface density of dihydride or trihydride species.