In previous work, it was noted that cations can form during adsorption of gases on metal surfaces. In this paper, high-resolution electron energy loss spectroscopy is used to determine whether pyridinium cations (C5H5NH+) can form on (2 x 1)Pt(110) under ultrahigh vacuum conditions. It is found that a new species forms when pyridine and hydrogen coadsorb on Pt(110) at 140 K. The new species has an HREEL spectrum that is consistent with the one expected for a pyridinium cation. There is also evidence that at low coverage, the molecular plane of pyridine is parallel to the surface at 100 K. At higher coverage, changes in the intensity of the in-plane versus out-of-plane vibrational modes show that adsorption phase transition occurs so that the molecular plane tilts with respect to the surface. These results are further confirmation of cation formation on metal surfaces.