Nuclear inelastic scattering (NIS) measurements were performed on a guanidium nitroprusside ((CN3H6)(2)[Fe(CN)(5)NO], GNP) monocrystal at 77 K after the sample was illuminated with blue light (450 nm) at 50 K to populate the two metastable states, MS1 and MS2, of the nitroprusside anion. A second measurement was performed at 77 K after warming up the illuminated crystal to 250 K where the metastable states decay to the groundstate. The measured spectra were compared with simulated NIS spectra that were calculated by using density functional methods. Comparison of measured and simulated spectra provides strong evidence for the isonitrosyl structure of the metastable MS1 state proposed by Carducci et al. (Carducci, M. D,; Pressprich, M. R.; Coppens, P. J. Am. Chem. Soc. 1997, 119, 2669-2678).