The adsorption of cytosine on an An electrode from 0.1 M NaCIO4 and HClO4 aqueous solutions has been investigated by in situ surface-enhanced infrared absorption spectroscopy and cyclic voltammetry. Two different spectra were obtained at potentials below and above 0.1 V versus SCE in the NaClO4 solutions. Similar behavior was observed also at 0.3 V versus SCE in the HClO4 solutions. From the spectral analysis, it was found that cytosine is physisorbed at negative potentials and is chemisorbed at positive potentials. Both the physisorbed and chemisorbed molecules are oriented with the N(3), C=O, and NH2 moieties directed towards the surface. A pair of redox peaks observed in the cyclic voltammogram were ascribed to the transformation between the physisorbed and chemisorbed states.