Abstract: Ultra-fine hollow glass spheres suitable as a filler in light-weight composite materials for building use and the like can be prepared from a fine powder of volcanic glass even when the starting powder has a particle size as fine as 20 .mu.m or smaller. The inventive method comprises an acid-leaching treatment of the starting powder using hydrochloric or sulfuric acid under hydrothermal conditions at 150.degree.-200.degree. C. to modify the chemical composition in the surface layer of the particles and a subsequent heat treatment of the acid-treated particles at a specified high temperature to effect expansion by the water vapor produced from the structural water in the softened particles. By virtue of the modified composition in the surface layer and in contrast to the prior art method without the acid treatment, a good balance can be obtained between the rate of water vapor release and softening of the particles even when the particle size is extremely small to facilitate expansion of the particles.