Two-dimensional vibrational spectroscopy is applied to the amide I mode of trialanine and two of its isotopomers dissolved in heavy water. We use site-directed C-13 isotope substitution to change the individual frequencies of the coupled oscillators, and hence to modify specific matrix elements of the molecular Hamiltonian. It is found that all of the results can be well described by an excitonic model for the amide I band, using the same coupling strength and dipole-dipole angle for all three isotopomers. This demonstrates that these two spectral parameters are determined by the secondary structure of the peptide, which remains unchanged upon isotope substitution. (C) 2001 American Institute of Physics.