<p>Polydimethylsiloxane (PDMS) membranes exhibit significant potential for use in CO₂ separation applications. However, the selectivity of CO₂ over N₂ and CH₄ in a pure PDMS membrane is relatively low. Herein, we developed mixed matrix membranes (MMMs) by incorporating a surface-treated fumed silica (FS) into a PDMS polymer matrix to enhance the CO₂ separation performance of PDMS membranes from N₂, and CH₄. The surface treatment of FS reduced its hydrophilicity, leading to more uniform dispersion within the PDMS matrix and increased fractional free volume by altering molecular packing, thereby enhancing CO₂ permeability as well as CO₂/N₂ and CO₂/CH₄ selectivity. Structural, morphological, and thermal stability analyses demonstrated that the PDMS matrix remained intact even with the incorporation of fillers at higher loadings. Single gas permeation tests revealed that the highest CO₂/N₂ and CO₂/CH₄ selectivity were achieved at FS loading of 7.5 wt%. The optimized MMM, containing 7.5 wt% fillers in PDMS, exhibited a 1.42-fold increase in CO₂ permeability and enhancements of 1.47-fold and 1.39-fold in CO₂/N₂ and CO₂/CH₄ selectivity, respectively, compared to neat PDMS. This study also indicates that feed pressure and time had no significant effect on CO₂/N₂ and CO₂/CH₄ separation performance.</p>