Dimethyldichlorosilane (DMDCS), diethyldichlorosilane (DEDCS), and dibutyldichlorosilane (DBDCS) were dissolved in supercritical CO2 at two concentration levels to modify hydrolyzed porous surfaces via silylation reactions. Plasma-damaged methylsilsesquioxane samples were loaded in a batch reactor with the chlorosilanes; when introduced, the low-viscosity supercritical CO2 dissolved and transported the chlorosilanes to the porous surface. Samples were characterized using Fourier transform infrared spectroscopy (FTIR), ellipsometry, goniometry, and electrical measurements, and compared against untreated samples. Reactions between the chlorosilanes and substrate hydroxyls were strongly dependent on the length of the alkyl group on the chlorosilane, but independent of concentration. FTIR analyses showed a decreased intensity for infrared (IR)-isolated/geminal OH vibrations (100%, 93.9% +/- 5.4%, and 95.4% +/- 4.3% for DMDCS, DEDCS, and DBDCS, respectively), but DEDCS and DBDCS resulted in 3.9% +/- 5.0% and 20.9% +/- 8.7% increases in hydrogen bonding, respectively. DMDCS was more successful, showing a 14.4% +/- 9.8% reduction in hydrogen bonding. Goniometry measured hydrophobic contact angles that were consistently greater than 81, irrespective of the chemistry used. Ellipsometry showed a strong dependence between the thickness of the deposited layers and the chlorosilane alkyl group (19.0 nm +/- 1.6 nm, 31.3 nm +/- 4.8 nm, and 74.2 nm +/- 4.7 nm for DMDCS, DEDCS, and DBDCS, respectively). Electrical measurements indicated improved hydroxyl group elimination with shorter alkyl groups (dielectric constant decreased from 3.5 for plasma-ashed methylsilsesquioxane samples to 2.59, 2.97, and 3.4 for DMDCS, DEDCS, and DBDCS, respectively). DMDCS was found to participate in intramolecular and intermolecular reactions while the surface-modifying agents with longer hydrocarbon chains underwent predominantly intermolecular reactions, resulting in the thicker deposited layers.