Controlled heating of single crystals of the previously reported [Köferstein & Robl (2007). Z. Anorg. Allg. Chem. 633, 1127-1130] dihydrate {[Co(pht)(bpy)(H₂O)₂]·2H₂O}_n, (II) [where pht is phthalate (C₈H₄O₄) and bpy is 4,4′-bipyridine (C₁₀H₈N₂)], produced a topotactic transformation into an unreported diaqua anhydrate, namely poly[diaqua(μ₂-benzene-1,2-dicarboxylato-κ²O¹:O²)(μ₂-4,4′-bipyridine-κ²N:N′)cobalt(II)], [Co(C₈H₄O₄)(C₁₀H₈N₂)(H₂O)₂]_n, (IIa). The structural change consists of the loss of the two solvent water molecules linking the original two-dimensional covalent substructures which are the “main frame” of the monoclinic P2/n hydrate (strictly preserved during the transformation), with further reaccommodation of the latter. The anhydrate organizes itself in the orthorhombic system (space group Pmn2₁) in a disordered fashion, where the space-group-symmetry restrictions are achieved only in a statistical sense, with mirror-related two-dimensional planar substructures, mirrored in a plane perpendicular to [100]. Thus, the asymmetric unit in the refined model is composed of two superimposed mirror-related “ghosts” of half-occupancy each. Similarities and differences with the parent dihydrate and some other related structures in the literature are discussed.