We investigate the photo-excitation process and the relaxation of the photo-excited state in photo-switchable solids, such as spin transitions systems. We follow the competition between the relaxation and the photo-excitation (up and eventually down) processes. The kinetic experimental curves are treated so as to derive the various contributions to the master equation, in terms of a single macroscopic variable: the population of the photo-excited state. The experimental examples are taken from [FexM1-x(btr)2 (NCS)2].H20, with M = Co, Ni, Zn. They give 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.