Abstract: Hollow glass spheres having average densities of approximately 0.10 grams/cc to approximately 2.0 grams/cc are prepared by heating solid glass particles. The glass spheres consist essentially of the following ingredients in the following amounts state as weight percentages: SiO.sub.2 (50 to 57%); R.sub.2 O(2 to 15%); B.sub.2 O.sub.3 (0 to 20%); S(0.05 to 1.5%); RO(2 to 25%); RO.sub.2 (other than SiO.sub.2) (0 to 5%); R.sub.2 O.sub.3 (other than B.sub.2 O.sub.3) (0 to 10%); R.sub.2 O.sub.5 (0 to 5%); and F(0 to 5%). R represents a metal or an element like phosphorous which combines with oxygen in glass. The sizes of the hollow glass spheres are selected to produce a maximum average strength for a desired average density.