Light-induced reactions of Si surfaces with carboxylic acids to generate Si ester-modified surfaces are studied. The reaction proceeds by photoelectrochemical oxidation of porous or (100)-oriented single-crystal n-type Si in formic, acetic, or trifluoroacetic acid electrolyte solutions, The reaction products at the porous Si surface are identified by Fourier-transform infrared (FTIR) spectroscopy, Derivatization with esters reduces the photoluminescence intensity of porous Si. X-ray photoelectron spectroscopy (XPS) of derivatized single-crystal Si is used to confirm the compositional and bonding information and to demonstrate that the same chemistry occurs at a single-crystal Si surface. A mechanism is proposed in which illumination of reverse-biased Si removes electron density from the Si surface, rendering Si-Si bonds susceptible to nucleophilic attack by carboxylic acid. The reaction has a marked dependence on light intensity and the Si surface can be photopatterned by illumination through a mask during derivatization, Ester-patterned porous Si reacts with CH3(CH2)(7)(CH3)(2)SiOCH3, generating an organosilane-patterned Si surface.