Using infrared spectroscopy and ab initio electronic structure calculations, we can make a conclusive assignment of the vibrational spectrum of pyridine adsorbed on W(110) and monitor its transformation to a-pyridyl. The assignments are further corroborated using isotopic N-15 vibrational shifts. We find that the pyridine molecule is already tilted edgewise prior to the transformation, and both the pyridine and the alpha -pyridyl species have their molecular planes perpendicular to the surface.