Photocatalytic TiO2 was derivatized with an amino silane to give a positively charged colloidal suspension in water. UV irradiation of the cationic titania oxidized the organo silane coating, converting the titania to a negatively charged colloid. Irradiation of mixed suspensions of cationic titania and cationic polyvinylamine microgel induced catastrophic flocculation. Electrophoresis and XPS evidence Suggests that the UV removed the amine groups from the titania resulting in heteroflocculation of anionic titania with cationic microgel particles.