The repair of porous methyl-substituted silicon dioxide films after plasma processing was investigated using hexamethyldisilazane (HMDS) dissolved in liquid and supercritical carbon dioxide. The films were cured, blanket methylsilsesquioxane (MSQ) layers (JSR LKD 5109). An ashed film treated with a HMDS/supercritical carbon dioxide (scCO2) mixture was stable for thirty days without readsorbing moisture from the ambient as shown by both Fourier transform infrared (FTIK) spectroscopy and contact angle measurements, whereas a film annealed to 360°C in nitrogen for 2 min readsorbed moisture quickly over 10 days. The film thickness increased by 2-5 nm and the surfaces were Hydrophobic after processing with HMDS. The dielectric constant of metal-insulator-semiconductor capacitors was 2.89 ± 0.08 in 1 vol% HMDS/liquid CO2, 2.55 ± 0.07 in 1 vol% HMDS/scCO2, and 2.57 ± 0.06 in pure liquid HMDS, demonstrating partial or complete repair after plasma ashing. Processing with supercritical CO2 yielded the best performance based on resistance to moisture readsorption, contact angle, dielectric constant, resource consumption, and processing time.