Spin-orbit coupling (SOC) in an Fe(II) spin-crossover system, [Fe(2-pic)(3)]Cl-2 . EtOH, where 2-pic denotes 2-picolylamine, is studied to understand the mechanism of light-induced excited spin state trapping (LIESST). The effects of SOC between the T-1(1)(T-1(2), E-5) states and the T-3(2) state are larger than those between the T-1(1)(T-1(2), E-5) states and the T-3(1) state. This suggests that the T-3(2) state can play a significant role in the LIESST and reverse-LIESST effects. (C) 2003 Elsevier Science B.V. All rights reserved.