In the course of the work of this thesis we have tried to describe the main mechanisms of the HS⟶LS relaxations after photo-excitation at low temperatures in spin-crossover compounds with the fundamental objective of understanding the processes that govern the apparition and growing of domains of centres either in the HS or in the LS state and their relationship with the cooperativity that takes place in these compounds during both, the light-induced and thermal-induced spin transitions. With this purpose in mind we have focused our work on the analysis of the structural evolution of two different systems during the HS⟶LS relaxation: The Hofmann clathrate [Fe(pz)Pt(CN)4], which is very well known for its excellent spin-crossover properties, since it shows a very cooperative transition around room temperature, and the [Fe(n-Bu-im)3(tren)](PF6)2 compound, for which two different thermal spin crossover behaviors have been observed depending on the scan rate of the temperature.