A self-assembled monolayer (SAM) Of L-cysteine [HSCH2CH(NH2)COOH] was prepared on a Au(1 11) surface by vapor deposition in ultrahigh vacuum and was characterized by techniques of temperature-programmed desorption (TPD), Cs+ reactive ion scattering (Cs+ RIS), and low-energy secondary ion mass spectrometry (LESIMS). Analysis of the amino acid functional groups of SAM indicated that L-CySteine molecules exist in the zwitterionic form. Upon physisorption of the D2O overlayer on the SAM, the -NH3+ functional group of cysteine readily exchanges their H atoms with D2O in the temperature range 125-230 K. The H/D exchange of the -NH3+ group sequentially occurs with D2O molecules that are directly hydrogen-bonded to the SAM, and the long-range proton transfer to the upper layer water molecules does not occur. Temperature-programmed reaction study and kinetic analysis yielded an activation energy of 13 +/- 1 kJ mol(-1) for the H/D exchange reaction, which suggests proton tunneling as a mechanism.