The performance of [emim][CH3COO] ionic liquid (IL) to separate mixtures of CO2 and H-2 is studied using both classical and ab initio simulation methods and experiments. Simulations show that H2 solubility and permeability in [emim][CH3COO] are quite low with Henry's law constants about 1 x 10(4) bar and permeabilities in the range 29-79 barrer at 313-373 K. In the case of CO2 absorption in [emim][CH3COO], ab initio molecular dynamics simulations predict two types of CO2 absorption states. In type I state, CO2 molecules interact with the [CH3COO](-) anion through strong complexation leading to high CO2 solubility. The C atom of CO2 is located dose to the O atoms of the [CH3COO](-) anion with an average distance of about 1.61 angstrom. The CO2 bond angle (theta(OCO)) is about 138 degrees, significantly perturbed from that of an isolated linear CO2. In type II state, the CO2 molecule maintains a linear configuration and is located at larger separations (>2.2 angstrom) from the [CH3COO](-) anion. The weaker interaction of CO2 with the [CH3COO](-) anion in type II state is similar to the one observed when CO2 absorbs in [bmim] [PF6]. Simulations further demonstrate that the [emim](+) cation competes with CO2 to interact with the [CH3COO](-) anion. The predicted high CO2 permeability and low H-2 permeability in [emim][CH3COO] are also verified by our experiments. The experimental CO2 permeability in [emim][CH3COO] is in the range of 1325-3701 barrer, and high experimental CO2/H-2 permeability selectivities of 21-37 at 313-373 K are observed. We propose that by replacing [emim](+) cation with 1-butyl-1-methylpyrrolidinium ([PY14](+)) further enhancement of CO2 solubility in [PY14][CH3COO] IL will be obtained as well as good performance to separate CO2 and H-2.