Studies have been carried out on two solvent-dependent structures of Co(NO3)2 with trans-1,2-bis(4-pyridyl)-ethylene (bpee) and their interconversions. X-ray analyses disclose that the molecule obtained in MeOH/CHCl3 is an infinite ladder structure, [Co(NO3)2(bpee)1.5·3CHCl 3)n (triclinic P1̄; a = 10.415(4) Å, b = 13.425(4) Å, c = 14.212(4) Å, a = 110.41(3)°, β= 109.06(3)°, γ = 97.20(3)°, Z = 2), whereas the compound afforded in H2O/EtOH is a mononuclear structure, [Co(H2O)4(bpee)2(NO3) 2·8/3H2O·2/3bpee (triclinic P1̄; a = 9.625(3) Å, b = 14.597(5) Å, c = 20.544(5) Å, a = 87.52(3)°, β= 83.34(2)°, γ = 75.94(3)°, Z = 2). For [Co(NO3)2(bpee)1.5· 3CHCl3]n, the local geometry around the cobalt(II) atom approximates to a heptacoordinate environment with four oxygens and three nitrogens. The four oxygen sites are occupied by an isobidentate and an anisobidetate nitrate ligand. Each bpee spacer connects two cobalt(II) atoms defining a T-shaped arrangement at the metal atom. For [Co(H2O)4(bpee)2](NO3) 28/3H2O·2/3bpee, the cobalt atom is in a typical octahedral arrangement with two bpee ligands and four water molecules. The bpee ligand is coordinated to the cobalt atom in a monodentate mode with a dangling pyridine group. Thermal and IR analyses of the present two complexes show that in the solid state the mononuclear complex is changed to the ladder structure at temperatures above 150 °C. Moreover, the ladder structure slowly shifts to the mononuclear complexes at sufficient moisture condition. Thus, either the ladder structure or the mononuclear complex exists depending on the presence or absence of moisture.