We show that the combination of a free running 80 MHz femtosecond Ti-sapphire oscillator with a nanosecond Nd:YAG laser makes it possible to record in real time, with a nanosecond time resolution, reversible or irreversible phase transitions photo-induced by a single laser pulse. This set-up is used to study the spin state transition photo-induced inside and outside the thermal hysteresis loops of a series of [Fe(NH(2)trz)(3)]X-2 center dot nH(2)O compounds (with X = NO3 and Br). Original dynamics are presented and discussed. The results indicate that at room temperature, it is the laser induced heating that drives the recorded photo-induced phase transition. (C) 2010 Elsevier B.V. All rights reserved.