Electrochemical sensors for copper ions in environmental samples were prepared by modifying gold electrodes with L-cysteine by self-assembly. The adsorption Of L-Cysteine on gold electrodes was studied by electrochemical reductive desorption in 0.5 M KOH, and the interaction Of L-cysteine with copper ions was investigated by cyclic voltammetry, chronoamperometry and X-ray photoelectron spectroscopy. At low concentrations the ratio Of L-cysteine to bound Cu(II) is 2:1. At higher concentrations (0.1 M) copper reacts with adsorbed cysteine forming copper sulfide on the electrode surface. On a modified L-cysteine gold electrode, Osteryoung square wave voltammetric determination of Cu(H) with a detection limit below 5 ppb has been demonstrated.