We report first results of quantum-chemical density functional theory calculations of the adsorption of formic acid on the Pt(1 1 1) surface from gas phase and in the presence of water molecules. We determine stable configurations of clusters HCOOH · (H2O)n, for n = 0,1,2 and their possible reaction products on uncharged and charged surfaces. The bonding of adsorbed molecules is analysed on the basis of charge transfer functions. All stable configurations of adsorbed formic acid in the presence of 0, 1 or 2 water molecules show the OCO plane of the molecule to be perpendicular or almost perpendicular to the metal surface. Stable configurations with the OCO plane parallel to the surface are not observed.