The melting of ordered monolayers of n-hexane, n-octane, and n-decane adsorbed on Pt(111) has been studied by reflection-absorption infrared spectroscopy (RAIRS). Each alkane forms an overlayer at low temperatures (<160 K) that is characterized by both quasi-long-range translational and orientational order. Previous low-energy electron diffraction (LEED) studies suggested that the alkanes are arranged on the surface in the all-trans conformation, and this has been confirmed by RAIRS. The low-temperature ordered state is distinguished by the presence of a sharp, resolved set of soft modes near 2760 cm(-1). The perturbation of the frequencies of some of the C-H oscillators due to the presence of M ... H-C interactions between the adsorbed n-alkane molecules and the surface has a dramatic effect on the RAIR spectra. Specifically, there are two RP;IR-allowed methylene upsilon(C-H) stretching modes : one which largely involves motion of the methylene C-H bond that projects away from the surface (the distal C-H bond) and another that largely involves motion of the near-surface (i.e., proximal) C-H bond. We have assigned these modes to the bands at similar to 2909 and similar to 2760 cm(-1), respectively.