The synthesis and structure of a completely new type of copper(i1) polymer [{[Cu,L,(dien)-( H20),]-3H20},], where L = 1 H-l,2,4-triazole-3,5-dicarboxylate(3-) and dien = diethylenetriamine (1.4,7-triazaheptane), is reported. The compound crystallizes in the tetragonal space group P42,c. with a = b = 15.845(3), c = 22.743(1) A and Z = 8. Least-squares refinement of 818 reflections with / > 2 0 ( / ) and 174 parameters gave R = 0.0694, R' = 0.0969. The polymer is essentially a double chain, the building blocks of which are trinuclear copper(ii) entities arranged zigzag clockwise and anticlockwise parallel to each other. The copper-copper distances within the trinuclear unit are Cu(1) Cu(2) 5.937(4), Cu(1) Cu(3) 4.014(6) and Cu(2) -.. Cu(3) 5.937(4) A. The two strands of the chain are linked to each other by apical Cu-0 bonds with Cu(2) 9 Cu(2') 3.498(6)A. The magnetic susceptibility data (5.8-284 K) are interpreted on the basis of an exchangeHamiltonian function giving J,[Cu(l)-Cu(3)] = -30.0 cm-l, J2[Cu(2)-Cu(2')] = -1.5 cm-l and g = 2.0. The nature and the strength of the antiferromagnetic exchange are discussed on the basis of the structural features, and compared with previously reported magneto-structural data for related copper(ii) compounds with N1,N2and N1,N4-linked 12.4-triazole ligands.