Hollow microspheres composed of organic-inorganic hybrid wall (polyorganosiloxane) have been synthesized as a new material in a W/O emulsion system from organosilanes. The average diameter of spheres for a typical run is 1.8 mum (standard deviation = 0.84) with a large spherical cavity in them and thermally stable at least up to 400degreesC in air. The polyorganosiloxane wall is amorphous and becomes porous by treating at around 400degreesC in air.