The pressure volume temperature composition relations in CO2 + ammonium bis(trifluoromethanesulfonyl)amide ionic liquid (IL) systems were measured over the pressure range up to 6 MPa at T = (298.15, 313.15, and 333.15) K. We focused on the effects of an ester or ether group in the cation of the IL on the CO2 solubilities on the mole fraction and molarity scales. N-Acetoxyethyl-N,N-dimethyl-N-ethylammonium ([N-112,N-2OCO1](+) and N,N-dimethyl-N-ethyl-N-methoxye-thoxyethylammonium ([N-112,N-2O2O1](+)) were the ester and ether functionalized cations, respectively. Their nonfunctionalized analogues N,N-dimethyl-N-ethyl-N-pentylammonium ([N-1125](+)) and N,N-dimethyl-N-ethyl-N-heptylammonium ([N-1127](+)), respectively, were also studied. Each IL shows the typical phase behavior as a physical absorbent. [N-112,N-2O2O1][Tf2N] and [N-1127][Tf2N] have the higher mole-fraction-scale solubilities of CO2 at certain temperatures and pressures, followed by [N-1125][Tf2N] and [N-112,N-2OCO1][Tf2N]. On the other hand, the molarity-scale solubilities of CO2 under the conditions investigated increase in the order [(N112,2OCO1)][Tf2N] < [N-1125][Tf2N] approximate to [N-1127][Tf2N] < [N-112,N-2O2O1] [Tf2N]. The ether functionalization in the ammonium cation is effective in the enhancement of physical absorption of CO2, in particular volumetrically, whereas the ester functionalization is negative.