This contribution reports the study of metastable spin states in spin-crossover materials in the framework of the atom-phonon coupling model. Using this model for a linear chain we show theoretically for the first time that, even if the cooperativity is not strong enough to obtain hysteresis at high temperatures, metastable states exist at low temperatures for a high spin (HS) fraction of [formula omitted]. This gradual thermal transition featuring metastable states at low temperature has been observed experimentally by [Létard et al., J. Phys.: Conf. Ser. 21, 23 (2005)]. Moreover, for compounds showing a two-step thermal transition, we show that metastable states, corresponding to a HS fraction [formula omitted], are present. This metastable states appear up to 50 K, fact that was observed by [Moussa et al., Phys. Rev. Lett. 94, 107205 (2005)] and [Matsumoto et al., J. Phys.: Conf. Ser. 148, 012029 (2009)]. © 2010, American Institute of Physics. All rights reserved.