Infrared reflection absorption (IRA) spectra were measured for methyl propyl ether (MPE) absorbed on Cu(110) and Ag(110) at 80 K. The IRA spectra of MPE on the substrates at sub-monolayer and full monolayer coverages exhibit simple spectral features, indicating that each adsorbate exists in a rotational isomeric form with a specific orientation relative to the substrate surface. Comparison of the IRA spectra of the adsorbates with those characteristic of MPE in bulk states taking known isomeric forms including TT, TG, GT, and GG forms (T and G are trans and gauche conformations, respectively, and the first symbol indicates the isomeric form around the O-CH2 bond and the second the form around the CH2-CH2 bond) and the simulation performed by using the density functional theory method for the IRA spectra of each isomer with varying orientations on a metal surface proved that MPE on Cu(110) takes on the GG form and MPE on Ag(110) the GT form. These forms are favorable for the coordination interaction of the oxygen atom to the metal surfaces. The spectral simulation indicated that the COC plane of the GG form tilts away from the substrate surface by about 30degrees to reduce the repulsion between the ethyl group and the Cu(110) surface, while the COC plane of the GT form tilts away from the surface by about 30degrees to a direction opposite of that of the GG form, resulting in the ethyl group with the C-C bond more or less parallel to the Ag(110) surface. From these results, it was concluded that the rotational isomerism and the orientation of the isomer on each surface are determined by the balance between the coordination interaction and an attractive (or van der Waals) interaction between the surface and the alkyl groups, the former interaction being more important on Cu(110) and vice versa on Ag(l 10).