Water adsorption on Pt(111) surfaces treated with oxygen or hydrogen chloride at 20 K has been studied by Fourier transform infrared spectroscopy and scanning tunneling microscopy. Water molecules chemisorb predominantly on the sites of the electronegative additives (O or Cl-), forming hydrogen bonds of O-H ... O or O-H ... Cl- On a Pt(111)-2 X 2-O surface, water adsorption produces species (O(D2O)), monomeric water (D2O), (O(D2O)(2)) and ring tetramer-like cluster (O(D2O)(3)) on a Pt(111) surface. On a Pt(111)-3 x 3-Cl- (theta = 0.44) surface, water adsorption gives rise to a Pt(111)-(4 X 2)(H3O+ + Cl-) co-adsorption structure to form a hydrogen-bonding network between Cl-and H3O+ ions.