Spin-crossover solids are photo-excitable materials, which are studied for future applications as optical memories or numerical display. The spin transition is a first order phase transition produced by variations of temperature, pressure or light. Cooperative effects constitute the basic mechanism for the spin crossover transition. They induce instability in the thermodynamic competition between the two states of the spin-crossover molecules: high-spin (HS) and low spin (LS).We investigate here the photo-excitation process and the relaxation of the photo-excited state in photo-switchable solids, such as spin transitions systems [(FexM 1-x)(btr)2(NCS)2] H2O, with M=Co, Ni, Zn. We follow the competition between the relaxation and the photo-excitation processes (up and eventually down). The experimental kinetics gives evidence for a non-linear character of the relaxation at all temperatures, including the tunnelling regime, and for non-linear behaviours of the photo-excitation rates.