We investigate here the consequence on light-induced and thermally induced spin-crossover (SCO) properties with particle size reduction from the macroscopic to microscale to nanoscale domains. Three samples with distinct particle sizes of the SCO coordination polymer [Fe(NCS)2(bpe) 2] [bpe = 1,2-bis(4'-pyridyl)ethane] have been prepared, by water-inoil reverse micelle methods. Comparison of the magnetic properties with particle size reduction of these and the original macroscale slow-grown crystals revealed that the spin transition becomes more gradual, more incomplete and concomitantly the transition temperature (T1/2) decreases -much like what is observed in metal dilution studies. Importantly, here, in the first photoinduced magnetic studies on a nanoparticle SCO system, we see that even on the nanoscale photoconversion of the low spin species to a metastable highspin state is possible. Furthermore, particle size reduction appears to have little effect on the temperature at which the stored photomagnetic information is erased. These results highlight that light-induced SCO properties are governed by direct metal coordination environment (i.e., on the molecular scale), whereas, thermally induced magnetic properties rely more on crystal, packing and ligand field effects. © 2010 Wiley-VCH Verlag GmbH & Co. KGaA.