The relative stability of the eta(1)mu(1) (atop) and eta(2)mu(2) (di-sigma) geometries of acetaldehyde are compared on Pt(1 11) and on two PtSn alloys ((2 x 2) and (root 3 x root 3)R30 degrees) by means of density functional theory (DFT) calculations. At low coverage on Pt (1/9 MQ, the two forms are equivalent in energy, with eta(1)mu(1) being slightly more stable. At high coverage (1/4 and 1/3 ML), eta(2)mu(2) is less competitive and acetaldehyde is adsorbed through the aldehydic hydrogen. The evolution of the adsorption energy with the coverage and the apparition of the structure adsorbed through the aldehydic hydrogen are explained by the existence of attractive dipole-dipole interactions. On PtSn, only the eta(1)mu(1) geometry is stable with an adsorption energy equal to that on Pt, in agreement with temperature-programmed desorption (TPD) experiments. The calculated vibrational spectra allow us to conclude that the experimental spectrum corresponds to a mixture of eta(1)mu(1) (majority) and eta(2)mu(2) (minority) structures on Pt and to only eta(1)mu(1) on PtSn. The various interactions and the relative stability of the species on Pt and PtSn are explained by the density of states (DOS) curves.