The effect of gamma ray irradiation on the density, refractive index, thermal expansion, and helium permeability of a variety of commercial borosilicate glasses has been measured for doses between 108and 1011rad. The radiation‐induced densification of these glasses for a constant dose can vary by as much as factor of 30 and appears to increase with increasing boron content of the glass. The refractive index also increases with dose and appears to be related to the change in density via the Lorentz‐Lorenz equation. The thermal expansion coefficient decreases with increasing dose for any given glass, and appears to vary inversely with the density change on a universal plot of change in thermal expansion coefficient versus change in density. Although the helium permeability was only measured for a few of the glasses, it appears that it is unaffected by radiation‐induced compaction of the glass. These effects are believed to result from the phase‐separated morphology of these glasses and from the less rigid nature of the network of one of these phases.