The thermodynamic systematics of the formation and phase changes of a range of materials which form ionic liquids is examined, based upon experimental values of densities and calorimetric quantities. Certain results are shown to be consistent across groups of these materials, namely: the lattice potential energies decrease with increasing alkyl chain length; the molecular volumes and 'total phase change' entropies (from crystal through liquid crystalline phases to melt) increase linearly with alkyl group chain length for pyridinium cations. However, the total phase change transition entropies for the two 1-alkyl-3-methylimidazolium cation systems examined are anomalous, having dome-shaped graphs with a dip in value around C-14. (C) 2004 Elsevier B.V. All rights reserved.