Chemical bond-changes of cholesteryl oleate in infrared free electron laser (IRFEL) photodynamic processes were examined by Raman spectroscopy and Fourier transform infrared microspectroscopy (FT-IR) as a function of the FEL exposure time. We found that the exposure of 5.79 μm-FELs induces not only dissociation of ester (RCOOR') bonds but also the chemical changes from the ester (RCOOR') bonds to aldehyde (RCHO), carboxylic acid (RCOOH), carboxylate (RCOO - ) or ketone (R 2 C O) bonds.