The structure of the title compound has been determined from X-ray diffractometer data by Patterson and Fourier methods. Crystals are monoclinic, with unit-cell dimensions: a= 13·971(2), b= 7·190(1), c= 19·900(3)Å, β= 101·82(2)°, space group P21/n, Z= 2. Least-squares refinement, by use of 2275 independent reflections, has reached R 0·060. The structure contains the centrosymmetric [Co(H2O)4(adenine)2]2+ ion, adeninium and sulphate ions, and molecules of water of solvation, held together by an intricate network of hydrogen bonds. In the complex cation the cobalt atom has octahedral co-ordination, and the co-ordinated adenine is unidentate, bonding viaN(9). Co–N is 2·164 Å, and the two independent cobalt–water distances are 2·073 and 2·114 Å. The hydrogen–bond network contains the following types of linkage: O–H O, N–H O, O–H N, N–H N, and possibly C–H O. A feature of the structure is the presence of adenine–adeninium pairs hydrogen–bonded together via N(1) H–N(10) and N(10)–H N(7).