Spirobenzopyran having a decamethylene spacer has been chemisorbed on the surface of colloidal silica particles of diameter 150 nm. The silica particles covered with the photochromic units displayed normal photochromism when they were dispersed in organic solvents having refractive indices matched with that of the silica. The fact that normal photochromism was observed even in highly polar ethylene glycol, in which a low-molecular-weight spiropyran shows reversed photochromism, indicates that the highly hydrophilic character of the silica surface is converted to a relatively hydrophobic one owing to the long hydrocarbon chain. When the photochromic particles were dispersed in carbon tetrachloride, the UV irradiation induced flocculation, causing sedimentation of the colloidal silica. Subsequent visible light irradiation recovered the dispersed state, showing the photocontrol of the dispersibility and offering a novel photoresponsive system.