Light induced surface reactions of reverse-biased n-type porous Si with H2O, CH3OH or CF3COOH are studied by diffuse reflectance and transmission Fourier transform infrared spectroscopy. The goal of this work is to identify reagents which will react with the irradiated, electron deficient surface but not the non-irradiated, neutral surface of reverse-biased porous Si. The porous Si is the anode in an electrochemical cell with the desired reagent present as the electrolyte solution. H2O/0.5M NaCl is found to react with the reverse-biased Si in both the light and the dark to form the native oxide. CH3COOH/0.1M NABF(4) generates a small amount of methoxide-modified Si in both the light and the dark. CF3COOH/1M CF3COONa reacts with the surface when the porous Si is optically excited to create a surface-bound trifluoroacetic ester species. CF3COOH/1M CF3COONa does not react with reverse biased Si in the dark. The Si surface can be 3 photolithographically patterned with the trifluoroacetic ester species.