We have used in-situ scanning tunneling microscopy and cyclic voltammetry to study self-assembled monolayers of ethanethiol on Au(lll) electrodes. The adlayer was found to consist of domains of two different ordered structures, one corresponding to a (p x root 3), well-established for other short-chain alkanethiols, and the other to an oblique primitive (4 x 3) superstructure, not previously reported for nonfunctionalized alkanethiols. At potentials slightly negative of 0 V vs SCE the adlayer undergoes a structural transformation that eventually leads to the formation of small pits and islands on the surface. Electrochemical studies in 0.1 M H2SO4 have revealed that around -0.31 V vs SCE the ethanethiol adlayer is reductively desorbed. Oxidative desorption of ethanethiol takes place at 1.15 V. The cathodic as well as the anodic desorption of the monolayer was monitored by scanning tunneling microscopy.