A new cyano-bridged polymeric mixed-valence CuII/CuI complex, [Cu(dien)2(CN)2Cu(CN)]n (2) (dien = diethylenetriamine) has been prepared by the partial reduction of the OH-bridged binuclear complex [Cu2(dien)2(OH)](ClO4)3 (1) with sodium cyanide. The X-ray crystal structure shows that 1 is a polymeric, linear mixed-valence CuI/CuII complex and consists of a one-dimensional three-coordinated [CuI(CN)]n chain, in which the CuI atoms are linked by two disordered CN bonding groups. The structure of each CuII atom consists of a square-pyramidal five-coordinate environment, in which the basal plane is formed by the three nitrogen atoms of the dien ligands and one carbon atom of the terminal cyanide group, and the apical position is occupied by the nitrogen atom of a bridging cyano ligand. This is the first example of one CuII centre coordinating with two different bonding types of the terminal CuII-CN and the bridging CuII-NC in a polymeric complex. The cryomagnetic behaviour has been investigated in the temperature range 4.2–300 K. There is a weak antiferromagnetic interaction between two intrachain copper(II) ions with J = −0.47cm−1.