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.