Fourier transform LR attenuated total reflection (FI?R-ATR) spectroscopy has been used to investigate the adsorption of acetic acid on the n-CdTe photoelectrode in aqueous and non-aqueous solutions. Electromodulated spectra associated with different adsorbates are observed. Acetonitrile as a solvent is adsorbed on the CdTe surface. A blue shift of nu(C=N) (2460 cm(-1)) is observed. Illumination affects the blue shift of the adsorbed acetonitrile. Current oscillations were observed under illumination in the anodic region of the cyclic voltammogram. Acetic acid, which is a bidentate ligand, is adsorbed on the CdTe surface. It reduces the blue shift caused by acetonitrile. The absorption intensities of the O-H bending and C-O stretching modes change under illumination. The photo-Kolbe reaction is present. In aqueous solution, the intensity of the C-O bending mode increases when the electrode potential becomes more positive. The adsorbent effects and electrochemical Stark effects related to the different adsorption models are discussed.