The preparation and crystal structure of the brown crystalline cobalt(III) dimer [(trenen)CoO2-Co(trenen)](ClO4)4 is described where the pentadentate ligand trenen is N, N, N′- tris (2-aminoethyl)ethane-1,2-diamine. The compound forms monoclinic crystals with a 13.132, b 14.214, c 8.906Â and β 91.53°, and two formula units in the unit cell, in space group P21/n. The nitrogen atoms of the trenen ligand occupy five of the six slightly distorted octahedral sites about each cobalt atom with a peroxo bridge linking the two Co(trenen) moieties through the sixth site. The Co-O-O-Co dihedral angle is 180°, the molecule possesses a centre of symmetry, and the peroxo bridging group is situated trans to the secondary nitrogen atom attached to each cobalt atom (the s, s -isomer). The electronic spectrum displays a strong (є 13700 dm3 mol-1 cm-1) absorption band at 302 nm which is typical of peroxo -bridged dicobalt complexes with a single transplanar CoO2Co bridge.