High-resolution electron energy loss spectroscopy has been used to study the coadsorption of water and carbon monoxide at the Pt3Ni(111) surface. The analysis of the vibrational spectrum indicates the reaction between coadsorbates (CO and OH groups of ice) with the formation of HOCO. Di-σ-trans-HOCO species are stabilized on the surface at up to 165 K. For higher temperature, HOCO desorbs from the surface.