The steric stabilization and flocculation of modified silica and alumina particle suspensions in condensed CO2 were studied. Silica particles (average diameters of 7 and 12 nm) were functionalized using chlorosilanes of the form CnF2n+1CH2CH2Si(CH3)(2)Cl (n = 8, 4, or 1) to give CnF2n+1-silica. Alumina particles (diameter of 8-14 nm) were grafted with C8F17CH2CH2Si(OEt)(3) and chemically modified with perfluorononanoic acid to yield C8F17-alumina and C8F17COOH-alumina, respectively. Elemental analysis and thermogravimetric analysis on the derivatized particles were carried out, and surface coverage was calculated. The stabilization of these modified particles in condensed CO2 was quantified using turbidimetry. Particle stability was found to increase with increasing fluorinated tail length, temperature, and CO2 density. Unmodified particles and those modified with only -CF3 tails were unstable in condensed CO2. Stabilization in supercritical CO2 is continuous up to 24 h for the CnF2+1-silica (n >= 4) particles and 96 h for the C8F17-alumina particles. The C8F17COOH-alumina particles gave a significantly higher graft density than the C8F17-alumina particles but are not as stable in CO2. The C8F17-alumina particles were stable at lower CO2 densities than the modified silica particles. This stability difference may be attributed to the precursor organosilanes being monofunctional (modified silica) versus trifunctional (modified alumina), producing different structures on the surface.