In order to deepen the understanding of cation-anion interaction in ionic liquids (ILs), the structure and interionic interaction of 1,1,3,3-tetramethylguanidinium lactate ([tmg][L]) ion pair, including stable configuration, hydrogen bond, frontier molecular orbital, electron density, ion interaction energy and charge transfer, are studied by using ab initio calculations. It is found that more charge-localized character of [tmg][L], especially the C1 carbocation on [tmg](+), and the intermolecular -NH2-associated hydrogen bonds can substantially increase the cation-anion interaction, the interaction energy is 65.3-109.3 kJ/mol higher than that of 1-n-butyl-3-methylimidazolium-based ILs. It is also found that the frontier molecular orbitals, i.e., the HOMO, HOMO+1 of [L](-) and the LUMO, LUMO + 1 of [tmg](+), can effectively interact and more charges are transferred between cation and anion. Based on the above results, the physical property of ILs is discussed. (c) 2007 Elsevier B.V. All rights reserved.