CO2 and CH4 capacities trapped by four solid-state ionic liquids, [N-2222][PF6], [N-4444][PF6], [C(12)mim][PF6], and [C(16)mim][PF6], were studied from 0.1 to 4 MPa at 303.15 K using a quartz crystal microbalance (QCM) measurement system. Two imidazolium ionic liquids with very long alkyl chains performed better in CO2 capture than the ammonium ones, while they were inferior for CH4 capture. Accordingly, [C(12)mim][PF6] and [C(16)mim][PF6] have higher CO2/CH4 selectivities than the ammonium ionic liquids. Due to the very long alkyl-substituted alkyl chain in [C(16)mim][PF6], ion aggregation makes it possess less space for gas accommodation, which is the main reason for its less CO2 and CH4 capacities than [C(12)mim][PF6]. The sorption simulation results also confirm that [C(12)mim][PF6] has larger CO2 and CH4 capacities than [C(16)mim][PF6]. In addition, molecular dynamics simulations were conducted using Lammps procedures, and it shows that the aggregation of cations of [C(16)mim][PF6] is stronger than that of [C(12)mim][PF6], leading to its less CO2 and CH4 accommodation space.