The structure of the title compound has been determined from three-dimensional X-ray diffractometer data. The crystals are monoclinic, with a= 11·134, b= 12·726, c= 10·404 Å, β= 119° 30′, Z= 2, space group p21/c. Full-matrix least-squares refinement, using 1187 independent reflections, has reached R 0·051. The complex, which has crystallographic symmetry, is trinuclear with two adenine molecules bridging three copper atoms via N(3) and N(9). Each adenine molecule spans a Cu Cu separation of 3·479 Å. The central copper atom is six-co-ordinate, with two N(3) atoms at 2·027 Å, and four chlorine bridging atoms forming two strong (2·313 Å) and two weak bonds (2·766 Å). The two terminal copper atoms are five-co-ordinate: Cu–N(9) 2·028 Å, Cu–Cl(terminal) 2·272 and 2·291 Å, and Cu–Cl(bridge) 2·324 and 2·743 Å. The co-ordination geometry is based on a square pyramid. A chlorine atom from an adjacent trinuclear unit occupies the sixth pseudo-octahedral position at 3·274 Å. Molecules of water of solvation, all non-donor nitrogen atoms of adenine and the chlorine atoms take part in a network of hydrogen bonds in which linkages of the type N–H O, N–H Cl, and probably O–H Cl are involved. The adenine molecule is protonated at N(1).