Using optical microscopy and various contrast methods we have investigated the spatio-temporal development of the two-step spin transition in single crystals of the [Fe(bapbpy)(NCS)(2)] complex. Nucleation and growth of domains displaying different molecular spin states were directly inferred from Raman spectroscopic images. The high-temperature transition proceeds in most cases by nucleation at a macroscopic defect, followed by growth along the c axis of the crystal. The development of elastic strain and associated birefringence are proved by polarized light measurements. The low-temperature transition is accompanied by the onset of twinning, which leads to a particular multi-domain growth mechanism. (c) 2010 Elsevier B. V. All rights reserved.