Reflectivity and magnetic measurements show that Co(CTH)(Phendiox)]PF6 . H2O (CTH = DL-5,7,7,12,14,14-9,10-hexamethyl-1,4,8,11-tetraazacyclotetradecane and Phendiox = semiquinonato or catecholato forms of 9,10-dioxophenantherene) undergoes valence tautomeric interconversion cobalt(III)-catecholato to cobalt(II)-semiquinonato upon light irradiation. This compound is the simplest cobalt-o-dioxolene complex undergoing this transition. The critical temperatures of the photoinduced transition and of the entropy driven transition seem to be related by the same empirical relationships observed for iron(II) spin crossover compounds. (C) 2004 Elsevier B.V. All rights reserved.