We have combined electronic and vibrational spec- troscopy in a cryogenic ion trap to produce highly resolved, conformer-selective spectra for the ground and excited states of a peptide containing two chromophores. These spectra permit us to determine the precise three-dimensional structure of the peptide and give insight into the migration of the electronic excitation from phenylalanine to tyrosine because changes in the excited-state infrared spectra are sensitive to localization of the electronic energy in each chromophore. The well-controlled experimental conditions make this result a stringent test for theoretical methods dealing with electronic energy transfer.