SAMs formed from 4-nitrothiophenol (4-NTP) on Au(lll) were studied in different electrolytes with in situ STM, cyclic voltammetry, and X-ray photoelectron spectroscopy. The 4-NTP SAM exhibits structural changes with the electrode potential, which coincide with the reduction steps of the nitro group. In general, at potentials where no faradaic processes occur, small domains (10 nm x 2 nm) with ordered structures are observed together with unordered phases, the latter ones occupying ca. 50% of the substrate surface. When subjecting the surface to negative potentials, the ordered structure disappears and monatomic-high islands covering 40% of the surface emerge, the nature of which could not be elucidated unequivocally. This starts at 20 mV versus saturated calomel electrode (SCE) in 50 mM H2SO4 and at -240 mV versus SCE in 50 mM Na2SO4. Upon further reduction, SAM reductive desorption is observed directly in scanning tunneling microscopy. This leads to a bare gold surface with characteristic triangular gold islands.