The glass‐formation region has been determined for the PbO–WO3–P2O5 system ‡ and certain physical properties of representative glasses have been measured. Glasses of the system show an unusually high X‐ray and gamma‐ray absorption and are not readily discolored when exposed to X rays. Equations have been developed from which specific volumes and lead equivalents may be calculated from composition. Because of their high index of refraction and relatively low dispersion these glasses are also useful for optical purposes.