The metastable states of the Cs[{Co(3-cpy)(2)HW(CN)(8)}]-H2O (3-cpy= 3-cyanopyridine) complex, characterized by a CTIST phenomenon with a high-temperature phase formed by Co-HS(II)(S = 3/2)-NC-W-V(S = 1/2) units and a low-temperature phase formed by H Co-LS(III) (S = 0)-NC-W-IV (S = 0) units, have been induced at low temperature either by thermal quenching (RC state) or by irradiation Is (PI state). The T(TIESST) and T(LIESST) relaxation temperatures of RC and PI states, respectively, have been determined and the incompleteness of photo-excitation discussed. The presence of sigmoidal kinetics and the observation of LITH hysteresis loop under irradiation demonstrate the cooperative nature of the relaxation. The thermodynamic parameters have been determined and a simulation of the T(TIESST) curve done. (c) 2006 Elsevier B.V. All rights reserved.