The formation of uracil adlayers on Ag(111) and Ag(100) in aqueous solutions was studied by cyclic voltammetry and capacitance-potential measurements. We found evidence for the existence of two distinctly different organized uracil layers at both crystal planes. The first is formed at medium coverages and potentials close to the potential of zero charge of the electrolyte E(pzc), and corresponds to a two-dimensional condensed physisorbed film. The second type of adlayer is formed at rather positive charge densities and shows extreme temperature stability. This film is assigned as chemisorbed uracil. The effect of the substrate on the properties of the physisorbed condensed film and the chemisorbed uracil layer is discussed in detail by comparing the results for the two silver single-crystal planes with those obtained for Au(lll), Au(100) and mercury. The following stability sequence was obtained for the physisorbed adlayer : Au(100)-(hex) > Au(111)-(px root 3) > Ag(100) > Ag(111) > Hg.