Dual frequency two-dimensional infrared (2D-IR) spectra were acquired with three IR fields, two at 3 and one at 6 mum from two pulses, creating a joint nonlinear response from both the amide-A (N-H) and the amide-I transitions of two model peptides AcAlaOMe and caprolactam. The 2D-IR spectra yield anharmonicities and correlations of inhomogeneous distributions. A positive correlation between the fundamental N-H mode frequency and its anharmonicity is found for the dipeptides. The amide-I frequencies are correlated with N-H mode frequencies and anticorrelated with the amide-I/N-H mode coupling. Structural implications of these results are discussed in relation to the anharmonic potential surface. (C) 2003 American Institute of Physics.