The interaction of a single formaldehyde or methanol molecule with a bare and a water-covered Pt(111) surface has been investigated using density-functional calculations, including the contribution of dispersion forces. van der Waals (vdW) terms are found to be essential to obtain the adsorption energies of both isolated molecules to the Pt(111) surface and the water wetting layer in agreement with experimental data. Moreover, the vdW correction changes completely the view on the mixed molecule-water-surface adsorption profile. In turn, the change induced by the vdW-forces on the geometric properties is very small for the minima found. For the water-covered surface, pure density-functional calculations underestimate the binding energy of a wetting-layer on Pt(111) by about 140 meV/H₂O molecule. The dispersion corrected calculations reveal a binding energy of 0.61 eV/H₂O molecule of a water bilayer at the platinum (111) surface, which is in perfect agreement with the experimental lattice energy of bulk ice. We show that the bilayer structure is, however, less stable by 93 meV/H₂O molecule than the ((37)^1/2 × (37)^1/2)-R25.3° periodicity reported in Phys. Rev. Lett. 2010, 105, 026102, that has recently been proposed as the first wetting layer on this surface. When adding both the molecules and the wetting layer, dispersion forces are the main contribution to the differentiation of the minima, particularly for a polar formaldehyde molecule interacting with the water-covered Pt(111) surface, for which the pure DFT predicts the adsorption in different structural motifs present in water environment to be energetically degenerated.