The solid state thermal isomerization of cis-[Fe(cyclam)Cl-2]Cl .H2O (cyclam is 1,4,8,11-tetraazacyclotetradecane), is investigated by TG/DSC, electrochemical and vibrational spectroscopic methods. The thermal isomerization. of this complex was found to be water dependent and endothermic. An aquation-anation pathway that precedes the macrocyclic ligand stereochemical rearrangement in the complex is proposed.