We show by variable-temperature X-ray diffraction and differential scanning calorimetry experiments that zinc imidazolates with coi and zni framework topology, respectively represent the thermodynamically stable phase below and above a transition temperature of ≈360 °C at ambient pressure. The relative stability of the two polymorphs and the experimentally observed strong negative thermal expansion of the coi phase at high temperatures close to the phase transition were successfully modelled using density functional theory calculations. A novel metastable zinc imidazolate with neb framework topology was detected by in situ X-ray diffraction experiments as a transient crystalline phase during solvothermal crystallisation of the stable coi phase. © 2013 The Royal Society of Chemistry.