Mössbauer spectroscopy has been extensively used as sensitive tool to determine oxidation state, coordination geometry and ordering of magnetic moments of iron in a variety of materials. In the specific case of a photoresponsive molecular polyanion prepared and characterized by Instituto de Ciencia Molecular (University of Valencia), Fe Mössbauer spectroscopy clearly showed that Fe(III) was reduced upon irradiation with UV light. The K salt of this material, in which two Fe(III) ions are simultaneously coordinated by two [A-α-PW9O34]9- polyoxometalate units and two oxalato ligands, when irradiated with UV light, exhibits a remarkable photocoloration effect due to the partial reduction of the POM units to give rise to a mixed-valence species. The process is reversible when UV light is switched off. The spectra show the material before (a) after irradiation (b), 24 h (c) and 5 days (d), after UV light switch off.