Using surface infrared absorption spectroscopy, we have measured and assigned the vibrational spectra of six different methoxy isotopotners adsorbed on a Cu(100) surface in great detail. The assignments are based on ab initio calculations of the vibrational mode frequencies of all six isotopomers for a methoxy-Cu-5 complex. By combining different IR detectors and optical filters, we observe many, previously undetected, weak spectral features, which we can assign as various binary overtone and combination modes. On the basis of these binary modes conclusions on the intramolecular anharmonic coupling in the adsorbate can be drawn.