Thermodynamic measurements on irradiated Na-2[Fe(CN)(5)NO]center dot 2H(2)O reveal the existence of four light-induced long-lived metastable states, which decay exponentially in time. The activation energies and frequency factors of the four independent states are determined from isothermal calorimetric and differential scanning calorimetric measurements. (c) 2004 Elsevier B.V. All rights reserved.