The Hoffman-type coordination compound [Fe(pz)Pt(CN)(4)]2.6H(2)O (pz=pyrazine) shows a cooperative thermal spin transition at around 270K. Synchrotron powder X-Ray diffraction studies reveal that a quantitative photoinduced conversion from the low-spin (LS) state into the high-spin (HS) state, based on the light-induced excited spin-state trapping effect, can be achieved at 10K in a microcrystalline powder. Time-resolved measurements evidence that the HSLS relaxation proceeds by a two-step mechanism: a random HSLS conversion at the beginning of the relaxation is followed by a nucleation and growth process, which proceeds until a quantitative HSLS transformation has been reached.