The adsorption and desorption of pyridine (Py) adlayers prepared on Au(111) terraces were studied by in-situ scanning tunnelling microscopy (STM) and conventional voltammetry using an aqueous 0.1 M HClO4 + 10(-4) M Py solution at 298 K. The applied potential covered the range 0.15 V < E < 1.2 V (vs. SHE), i.e. potentials above and below E-pzc, the potential of zero charge of Au(111). In the range E-pzc < E < 0.96 V, hexagonal (4 x 4) ordered domains corresponding to Py molecules adsorbed vertically on the Au(111) surface coexist with disordered adlayer domains, but for E < E-pzc, the ordered adlayer structure disappears leaving uncovered Au(111) domains. Similarly, for E > 0.96 V, the (4 x 4) adlayer lattice is removed completely leading to a bare Au(111) surface. By stepping E backwards to a value in the range E-pzc < E < 0.96 V, the readsorption of Py takes place and the (4 x 4) adlayer domains are recovered in a few minutes. STM data offer the possibility of discussing anion adsorption and the early electroformation stages of the OH-containing adlayer on Au(111) terraces.