The interaction and reactivity of n-octane adsorbed on the (1 x 1) and (5 x 20) surfaces of Pt(100) have been examined by reflection-absorption infrared (RAIRS), temperature-programmed reaction (TPRS), and Auger electron (AES) spectroscopies and by low-energy electron diffraction (LEED). A strong C-H ... M interaction between the adsorbate and the metal was present on both surfaces at 100 K, as evidenced by the presence of "softened modes" in the C-H stretching region of the vibrational spectra centered at similar to 2630 and 2750 cm(-1) for the (1 x 1) and (5 x 20) surfaces, respectively. The softened modes observed for n-octane on the (5 x 20) surface of Pt(100) are reminiscent of those seen when this molecule is adsorbed on Pt(lll). On both of these surfaces (Pt(100)-(5 x 20) and Pt(111)) the molecule adopts an all-trans conformation and is adsorbed so as to align the plane of the C-C-C framework parallel to that of the surface. This organization leads to a series of bands appearing in the 2500-2840-cm(-1) region which result from the high-symmetry C-H ... M contacts occurring between the n-alkane overlayer and the underlying hexagonal symmetry surfaces. The softened modes observed for an overlayer of octane on the unreconstructed (1 x 1) surface at low temperature, however, were broad and featureless. Relative to the case for the Pt(100)-(5 x 20) surface, the activity for the dehydrogenation of an overlayer of n-octane was much greater on Pt(100)-(1 x 1). These reactivity differences appear to be weakly correlated with the nature of the mode-softening seen in the low-temperature vibrational spectra.