The C-H stretch and bend modes of ethoxy adsorbed on Cu(100) are investigated using Fourier transform infrared spectroscopy. Three fundamental stretch modes and four fundamental bend modes are assigned. In addition, five binary bend modes are identified; two combination and three overtone modes. It is demonstrated that by combining quantum mechanical ab initio electronic structure calculations and C-13 isotopic labeling, a conclusive assignment of all observed modes is possible. The implications for the determination of surface/adsorbate geometry using vibrational spectroscopy are discussed.