Using two complementary techniques, the relaxation of a photo-induced spin state transition in a strongly cooperative compound has been studied from 10 to 170 K. Up to 150 K, the relaxation rate increases with temperature. At higher temperature, the stabilization of the high-spin state results in a decrease of the relaxation rate. Within the hysteresis loop, as one approaches the switching temperature branch, a critical slowing down of the kinetics and a constant decrease of the cooperative contribution are recorded. This study evidences, for the first time, the complex temperature evolution of the relaxation of photo-induced spin state transition. (c) 2005 Elsevier B.V. All rights reserved.