Titanium dioxide nanoparticles can be produced by the controlled hydrolysis of titanium tetraisopropoxide in water-in-CO₂ (w/c) microemulsions stabilized with the surfactants ammonium carboxylate perfluoropolyether (PFPE-NH₄) and poly(dimethyl amino ethyl methacrylate-block-1H,1H,2H,2H-perfluorooctyl methacrylate) (PDMAEMA-b-PFOMA); the greater control of hydrolysis and particle growth with PDMAEMA-b-PFOMA is consistent with the differences in the stabilities and interactions for these two microemulsions.