The effect of X-ray illumination on the structural properties of the mixed valence Prussian blue analogue CsFeII[CrIII(CN) 6] has been studied by time-dependent high-resolution synchrotron X-ray diffraction. Abrupt isosymmetric phase transitions, accompanied by dramatic volume collapse, were found in the temperature range 245-265 K, induced by sudden FeII spin transitions from the high spin (HS) ( 4t2g2eg, S = 2) to the low spin (LS) (6t2g0eg, S = 0) configuration. Absorption of X-ray photons generates photoexcited FeII(LS) domains whose size rapidly grows with time until the percolation threshold is reached and the structure collapse is triggered. The persistent character of the optically excited spin crossover states derives from the strong electron-phonon coupling, associated with the large lattice relaxations, which accompany the internal spin rearrangements. It is thus possible to use X-ray light in a controllable and efficient way to induce photoswitching between the ground and hidden or inaccessible excited states in suitably selected multistable materials in the bulk.