Nanoparticles exhibiting a cooperative spin transition, prepared using the reverse micelle technique, are reported. A 40 K magnetic hysteresis is observed near room temperature for [Fe(Htrz)(2)(trz)](BF4) nanoparticles (where trz is a triazole derivative) of 11 +/- 5 nm in diameter. The figure shows the nanoparticles in octane in the low-spin state (left) and the high-spin state (right).