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Abstract: Iron mixed-valence complex, (n-C3H7)4N[FeIIFeIII(dto)3] (dto=C2O2S2), shows a spin-entropy driven phase transition called charge transfer phase transition around 120K and a ferromagnetic transition at 6.5K. These phase transitions remarkably depend on the hexagonal ring size in two-dimensional honeycomb network structure of [FeIIFeIII(dto)3]∞. In order to control the magnetic properties and the electronic state in the dto-bridged iron mixed-valence system by means of photo-irradiation, we have synthesized a photo-sensitive organic–inorganic hybrid system, (SP)[FeIIFeIII(dto)3] (SP=spiropyran), and investigated the photo-induced effect on the magnetic properties by the measurements of magnetic susceptibility, ESR, IR, UV–vis and 57Fe Mössbauer spectra. Upon UV irradiation at 337nm, a broad absorption band between 500nm and 600nm appears and continuously increases with the photo-irradiation time, which implies that the UV irradiation changes the structure of spiropyran from closed form to open one in solid state. The photochromism in spiropyran changes the ferromagnetic transition temperature and the coercive force. [Copyright &y& Elsevier]