The isothermal conductivity and solubility of CO2 in amorphous poly(oligo-oxyethylene glycol methacrylate) (PMEO)-LiX (X = N(SO2CF3)(2), CF3SO3, LiClO4, BF4, and Br) electrolytes at CO2 pressures between 0.1 and 20 MPa were measured by an in-situ impedance unit consisting of a supercritical CO2 extraction system and an original high-pressure reactor. Solubility was estimated from the gravimetric change at desorption procedure (CO2-out), which is based on the Fickian diffusion of CO2 molecules in polymer. The relation between the conductivity in CO2 and q(t=0) (the amount of saturated CO2 in 1 g of a sample) is discussed as a function of pressure. The conductivity in pressurized CO2 increases linearly with increasing q(t=0) due to the decrease in T-g and the effect on q(t=0) of fluorine atoms in the anion; there is the Lewis acid-base interaction between CO2 molecules and dissociated anions. The LiTFSI electrolyte had the highest q(t=0), more than 0.35, and the conductivity at 20 MPa was 17 times higher than that at 0.1 MPa. In-situ FT-IR measurements revealed that the CO2 molecules absorbed into the PMEO matrix weaken the original CO/OH hydrogen bonds and enhance the flexible ether side chains leading to fast ionic transport. Studies on solid polymer electrolytes (SPEs) such as PMEO using pressurized CO2 fluid as a treatment medium have a possibility to realize excellent ionic conductivity even in the dry state. We expect that a novel preparation process for SPEs can be established using the CO2 as solvent, which leads to an environmentally benign system for sustainable chemistry.