Light-induced excited spin state trapping of Fe(phen) 2 (NCS) 2 is studied by X-ray absorption spectroscopy using synchrotron radiation. The molecular structure of the excited high-spin state at 17 K is investigated by EXAFS. The relaxation of the light-induced high-spin state at various temperatures is monitored through the absorption spectra. The concurrent abrupt lattice change with the change in electronic configuration is observed through single crystal X-ray diffraction. The lattice cooperative effect is important in the relaxation behavior. The magnetic property and the relaxation behavior of two differently prepared samples are distinctly different.