The fifth order contributions to the signals of ultrafast infrared spectrally resolved stimulated vibrational echoes at high intensities have been investigated in carbonmonoxy heme proteins. High intensities are often required to obtain good data. Intensity dependent measurements are presented on hemoglobin-CO (Hb-CO) and a mutant of myoglobin, H64V-CO. The spectrally resolved vibrational echoes demonstrate that fifth order effects arise at both the 1-0 and the 2-1 emission frequencies of the stretching mode of the CO chromophore bound at the active site of heme proteins. Unlike one-dimensional experiments, in which the signal is integrated over all emission frequencies, spectrally resolving the signal shows that the fifth order contributions have a much more pronounced influence on the 2-1 transition than on the 1-0 transition. By spectrally isolating the 1-0 transition, the influence of fifth order contributions to vibrational echo data can be substantially reduced. Analysis of fifth order Feynman diagrams that contribute in the vibrational echo phase-matched direction demonstrates the reason for the greater influence of fifth order processes on the 1-2 transition, and that the fifth order contributions are heterodyne amplified by the third order signal. Finally, it is shown that the anharmonic oscillations in vibrational echo data of Hb-CO that previous work had attributed strictly to fifth order effects arise even without fifth order contributions. (C) 2004 American Institute of Physics.