A procedure has been developed for the synthesis of silica-supported polymerization initiators, which includes chemisorption of (3-aminopropyl) triethoxysilane, hydrolysis of the alkoxy groups, and chemisorption of 4,4′-azobis(4-cyanopentanoic acid). It has been found that the reaction of 4,4′-azobis(4-cyanopentanoic acid) with immobilized amino groups in the presence of N,N′-dicyclohexylcarbodiimide involves only one functional group in the former. The effect of the solid support porosity on the composition of organosilica obtained by surface-initiated radical polymerization of acrylic acid has been studied. The best yield of grafted polyacrylic acid is obtained on aerosilogel with large mesopores in combinations with developed surface. The dependence of the impedance of polyacrylic acid-modified aerosilogel on the relative air humidity has been studied, and the possibility of using this material in humidity sensors has been shown.