Nanoparticles of the spin crossover complex [Fe(NH₂trz)₃](tosylate)₂ (tosylate = p-toluene sulfonate anion) were synthesized and dispersed in a non-ionic surfactant through a microemulsion-based method. Dynamic light scattering and transmission electronic microscopy was used to determine the mean sizes of the particles, which can be tuned from 3–4 nm to 40 nm depending on the synthetic conditions. The thermal spin crossover behavior remains cooperative and appears only weekly dependent on the particle size within this size range. The thermal treatment of the colloidal suspensions leads to the transformation of the particles into fibrous structures. On the other hand, the destabilization of the microemulsions by an adequate solvent leads to a powder with larger particles, which display properties similar to the bulk material.