Organic Ionic Plastic Crystals (OIPCs) are solid-state analogues of ionic liquids, but maintain a crystalline matrix at room temperature. The materials also exhibit multiple solid-solid phase transitions with increase in temperature owing to internal molecular motions and weak interionic interactions. They have been largely explored as solid-state electrolytes for battery applications. Recently, McDonald et al. have reported, with experiments, an ideal selectivity equal to 30 for the CO2/N2 mixture at 35 °C using the OIPC methyl(diethyl)isobutylphosphonium hexafluorophosphate ([P122i4][PF6]) (Chem. Commun., 2016, 52, 12940). Here we investigate gas absorption in the phosphonium based OIPC using simulations to gain atomic-scale insights on the absorption mechanism. Ab initio methods were used to assess the interaction strength between the ions and gas molecules. Classical molecular dynamics (MD) simulations were carried out to study gas absorption at temperatures corresponding to different plastic crystal phases by employing a gas/solid interface model. CO2 showed the highest absorption compared to CH4, O2 and N2. Furthermore, the CO2 absorption was dependent on both the temperature and the degree of structural ordering in the material. This work demonstrates that the OIPC could be promising in separating CO2 from a number of different gases.