Recently, the guanidinium-based ionic liquid N,N,N 0 ,N 0 ,N 00 -pentamethyl-N 00 -propylguanidinium tetrafluoroborate ([ppg][BF 4 ]) was found to be able to effectively absorb CO 2 (Amitesh, M. ChemSusChem 2009, 2, 628). In this work, molecular dynamics (MD) simulations and ab initio calculations were performed to investigate the microscopic structure, interactions, and properties of pure ionic liquid [ppg] [BF 4 ] and mixtures of [ppg][BF 4 ] (1) and CO 2 (2) at x 2 = 0, 0.1, 0.3, 0.5, 0.6. CÀH 3 3 3 F hydrogen bonds were found to exist between [ppg] cations and [BF 4 ] anions by ab initio calculations. However, the H 3 3 3 F coordination numbers suggest that the H 3 3 3 F interaction decreases slightly with increasing CO 2 concentration. CO 2 molecules were found to be primarily distributed around the anions, as seen from the CO 2 Àcation and CO 2 Àanion radial distribution functions (RDFs) calculated from molecular dynamic simulations. The cationÀanion and anionÀanion structures were not perturbed even with addition of CO 2 up to 0.6 mol fraction, whereas the cationÀanion and anionÀanion interactions decreased with increasing CO 2 concentration. The volume expansion and diffusion of [ppg] [BF 4 ] increased with increasing CO 2 concentration. In addition, according to the results of RDFs and spatial distribution functions, the distribution areas of CO 2 molecules and [ppg] cations around [BF 4 ] anions were found to be nearly complementary.