The spin-crossover compound [ Fe(bbtr)(3)](ClO4)(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 13 K wide hysteresis around 105 K, 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(down arrow) 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. (C) 2008 Elsevier B.V. All rights reserved.