Surface-relief photonic gratings of the spin-crossover metal-organic framework {Fe(bpac)[Pt(CN)4]} (bpac = bis(4-pyridyl)acetylene) were elaborated by the combination of a sequential assembly process and lithographic methods. Optical diffraction, surface plasmon resonance spectroscopy and Raman micro-spectroscopy were used to investigate the temperature dependence of the spin state of the iron(II) ions and the concomitant change of the refractive index of the grating material. The refractive index change associated with the high spin ((5)T) to low spin ((1)A) transition was found to be as high as Δn = 0.08 ± 0.005, which was attributed to the pronounced mass density difference between the two spin states. While the grating thickness (15-90 nm) had no influence on the spin-crossover properties of the gratings, the adsorption of aromatic guest molecules was found to have a substantial effect both on the spin transition temperature and the completeness of the transition.