In this work, the study on the volatility of ionic liquids is focused on the 1,1,3,3-tetramethylguanidinium-based ionic liquids. Vaporization enthalpy and cluster species in gas phase for 1,1,3,3-tetramethylguanidinium lactate ([tmgH][L]), 1,1,3,3-tetramethylguanidinium trifluoroacetate ([tmgH][T]), and 1,1,3,3-tetramethylguanidinium formate ([tmgH][F]), are investigated by using molecular dynamic simulation and ab initio calculation, respectively. Results from the molecular dynamic simulations show that the interionic interactions of coulombic electrostatic and Van der Waals forces are the main factors for deciding the volatility. In addition, owing to the change of molecular conformations from the liquid phase to the gas phase, intraionic bond, angle, and torsion interactions also give remarkable contributions. From the ab initio calculations, in the gas phase, an interionic proton transfer easily occurs in the ion pairs of these guanidinium-based ionic liquids, and the ion pairs are finally transformed into more thermodynamically stable neutral molecule dimers (this is different from some imidazolium-based ILs where ion pair can stably exist in gas phase). The transfer energy barriers are very low (typically, less than 2 kJ mol(-1)). However, the existence of a third charged ion ([tmgH](+), [L](-), [T](-), or [F](-)) or neutral molecule (tmg, HL, HT, or HF), will stabilize the ion pairs and prevent the transfer of proton. Finally, the stable trimers are then formed. The tetramers are also stable species. Ab initio results explain why they exist as ions in the liquid state. (C) 2010 American Institute of Chemical Engineers AIChE J, 57: 507-516, 2011