Mössbauer spectroscopy as a function of temperature of Fe(II)(N,N′-dicyclohexyl-thiourea)6(ClO4)2(FeDCTU) shows the existence of two isomeric forms A and B which coexist in the temperature range 200 < T < 300 K. The relative concentrations of A and B were studied as a function of time at different temperatures in the coexistence region. From the equilibrium constant as a function of temperature we obtain the following thermodynamic parameters: ΔH = −4520 ± 120 cal/mol and δS = 20 ± 0.1 e.u. The kinetics seem to follow an ordinary first order law but with an unusual temperature dependence of the rate constant k1. The results are discussed in terms of hydrogen bonded interactions between the perchlorate anions and the cyclohexylthiourea moieties.