Direct sol-gel reactions in supercritical CO2 (scCO(2)) have attracted significant interest for synthesizing nanomaterials by reacting alkoxides with a carboxylic acid. In this study, the hydrolysis of silicon alkoxides (TEOS or TMOS) was carried out using ScCO2 as the solvent to generate silica nanoparticles within the matrix of polyethylene for the synthesis of polymeric nanocomposites. This methodology provides advantages of combining the sol-gel reactions and drying into a one-step process for producing polymer nanocomposites. The synthesized polymer silica composites were characterized by SEM, FTIR, and XPS. When the TEOS loading was below 10 wt % Si content, nanometer-sized silica particles were formed that were well dispersed within the polyethylene matrix. The mechanism of the silicon alkoxides reacting with acetic acid in SCCO2 was further studied using online GC-MS and offline C-13 NMR. Oligomer structures with a bridging methoxyl group between the two silicon atoms and the acetate monodentate were observed. This study suggests a new sol-gel pathway in scCO(2) that is different from the hydrolysis-condensation reactions using the conventional sol-gel process.