The adsorption of uracil on Au(100) has been studied by in-situ scanning tunneling microscopy (STM). Defending on the potential applied to the gold electrode? uracil forms three highly ordered monolayers, which were imaged with molecular resolution. "Steady state" and time-dependent experiments were performed to follow structural changes of the respective substrate surface in the presence of these organic layers. A physisorbed film of planar oriented and via hydrogen bonds connected molecules exists at negative electrode charges. Coupled with changes of the electronic state of adsorbed uracil this monolayer undergoes a first order phase transition when altering the electrode potential towards more positive values. A third (chemisorbed) adlayer is finally formed simultaneously with the substrate surface structural transition (hex) --> (1 x 1). Uracil changes its orientation from planar to perpendicular, and a substrate specific surface coordination complex is created. The "equilibrium structure" is determined by the competing action of substrate-adsorbate coordination and lateral stacking interactions.