The electrochemical behavior and surface chemistry of anodic silicon etching in nonaqueous electrolytes was studied. Etching of single-crystal p-type and n-type (100) silicon was carried out in acetonitrile and propylene carbonate with hydrofluoric acid (HF) or tetrafluoroborate (BF4-) providing fluoride to complex the oxidized silicon. Electrolytes containing HF resulted in tetravalent dissolution, and photocurrent quadrupling was observed. Electrolytes containing BF4- also resulted in tetravalent dissolution; however, calculated quantum efficiencies were lower depending upon the electrolyte. Current-voltage behavior indicates the presence of surface states which affect both the onset potential for oxidation and the current multiplication. In situ multiple internal reflection Fourier transform infrared analysis confirms that silicon surfaces etched in electrolytes containing HF remain hydride-terminated throughout etching; however, silicon etched in BF4- based electrolytes loses the initial hydride termination at the onset of etching.