The spin-crossover compound [Fe(bbtr) 3 ](ClO 4 ) 2 (bbtr=1,4-di(1,2,3-triazol-1-yl)butane) forms a polymeric hexagonal sheet structure. It shows an abrupt thermal spin transition with 13K wide hysteresis around 105K, as evidenced by single crystal optical spectroscopy. The transition temperature for the thermal high-spin→low-spin transition on cooling as well as the relaxation kinetics just below T c ↓ depend upon the history of the sample. This is typical for a nucleation and growth mechanism and domain formation. In contrast, the high-spin→low-spin relaxation following the light-induced population of the high-spin state at low temperatures is governed by the intersystem crossing process.