The adsorption of pyridine (Py) on Au(111) terraces produced from aqueous 0.1 M HClO4 + 10(-3) M Py at 298 K was studied in the potential range 0.15 V < E < 0.55 V (vs SHE) by in-situ scanning tunneling microscopy. When E greater than or equal to E-pzc, the potential of zero charge of the substrate, both ordered and disordered domains can be observed. Ordered domains correspond to vertically adsorbed Py molecules forming a (4 x 4) hexagonal lattice with the nearest neighbor distance d = 0.38 nm, and corrugation z = 0.04 nm. The ordered adsorbate structure disappears for E < E-pzc, but it is recovered several minutes after stepping E backward to E > E-pzc, as expected for a reversible potential-step-induced surface process. The surface concentration of adsorbed Py molecules resulting from the (4 x 4) lattice is close to 1 x 10(-9) mol/cm(2), a figure exceeding the average integral surface concentration value obtained from electroadsorption measurements. Therefore, disordered domains would be related to a mobile diluted Py adsorbate coexisting with ordered Py adsorbate domains leading to a nonhomogeneous Py adsorbate layer at the positively charged Au(111) terraces.