Photomagnetic effects were recently evidenced in CoFe Prussian-blue analogues. In order to understand the mechanism and the scope of the phenomenon, we undertook the study of the interaction between light and matter in one selected compound and the study of the relationship between structure and physical property in the family of compounds of chemical formula X_xCo₄[Fe(CN)₆]_(8+x)/3 where X is an alkali cation. The first part is devoted to the characterization of the metastable excited state by X-ray absorption spectroscopy and X-ray diffraction. Photomagnetic properties in CoFe Prussian blue analogues are due to the photo-induced electron transfer from Fe^II(LS) to Co^III(LS) in Fe^II-Co^III diamagnetic pairs. It is accompanied by the bond lengthening of the cobalt to ligands bonds and the lengthening of the cell parameter of the fcc phase. The second part is devoted to the study of the photomagnetic properties of X_xCo₄[Fe(CN)₆]_(8+x)/3 compounds in which x, the number of alkali cation per cell, varies. We present here how the photomagnetic Fe^II-Co^III species are formed during the synthesis of the compounds and the change of the photomagnetic properties of the diamagnetic pairs when the surrounding inorganic network varies. To cite this article: V. Escax et al., C. R. Chimie 6 (2003).