The crystal and molecular structure of the title complex has been determined by X-ray photographic methods. The unit cell is trigonal, with a=b= 18·05(1), c= 7·25(1)Å, space-group P, and Z= 2. The structure was solved by conventional Patterson and Fourier methods, and was refined by least-squares to R 0·15 for 2043 unique reflections. Only a few of the water molecules of crystallization could be located. The crystal is composed of trinuclear units of copper atoms each centred about a three-fold crystallographic axis so that the three copper atoms fall at the corners of an exact equilateral triangle of side 3·22 Å. The copper atoms are held together by three distinct bridging systems: (i) the oxygen atom of the hydroxy-group, situated on the three-fold axis, is bonded to all three copper atoms with Cu–O 1·98 Å, (ii) a sulphato-group, also on the three-fold axis, acts as a tripod bridge, bonding to all three copper atoms through three of its oxygen atoms, and (iii) three symmetry-related pyridine-2-carbaldehyde oximato-groups each functioning both as a bidentate chelate to one of the copper atoms (via both N atoms) and as a Cu–Cu bridging group (via oxime N and O). The trimer units are stacked in continuous columns parallel to the c axis and adjacent units are held together by hydrogen bonding. These columns are arranged such that a large almost cylindrically shaped hole (diameter ca. 10 Å) is formed, running along the length of the c axis, and containing the molecules of water of crystallization; these are not held in fixed positions but appear to be somewhat mobile. The structure of the complex confirms a prediction made on the basis of the observed temperature-independent magnetic moment of 1·0 B.M. per copper atom.