Infrared reflection absorption spectra were obtained from 900 to 3000 cm(-1) for five C-3v symmetry isotopomers of methoxy: (H3CO)-C-12-O-16, (H3CO)-C-13-O-16, (H3CO)-C-12-O-18, (D3CO)-C-12-O-16, (D3CO)-C-13-O-16; and two C-s symmetry isotopomers: (DH2CO)-C-12-O-16, and (HD2CO)-C-12-O-16, Measured frequencies of the five C-3v isotopomers were used to derive an empirical valence force field based on a C-3v symmetry H3COCu3 model. Without further adjustment, this force field was used to calculate the harmonic frequencies of the two C-s symmetry isotopomers which were found to be in good agreement with the experimental frequencies. Comparison of a C-s symmetry H3COCu12 model with an adsorption site structure that accurately models the experimentally determined geometry, as well as with a C-3v symmetry H3COCu model, shows that the internal vibrations of the methoxy unit are relatively insensitive to the adsorption site. Among other advantages, an accurate description of the normal modes of this molecule provides a quantitative understanding of the unusual intensity patterns in the CO stretch and CH3 symmetric deformation (delta(s)(CH3)) region, which leads to an extremely low intensity for the delta(s)(CH3) fundamental for the (H3CO)-C-12-O-16 isotopomer. In addition to the vibrational properties of methoxy that can be understood within the harmonic approximation, two manifestations of anharmonicty are observed: (1) the position of the overtone of the CO stretch; and (2) Fermi resonance in the CII stretch region between the symmetric CII stretch fundamental and the overtones of the symmetric and asymmetric CH3 bends. The normal coordinate analysis allows the position of the unperturbed vibrations in the CR stretch region to be more accurately estimated and helps to explain the weaker Fermi resonance in the CD stretch region of D3CO, finally, the empirical valence force field is compared with the force field derived from an ab initio calculation of H3COCu.