Water introduced in silica reacts with the silica network under hydroxyl generation that causes a volume swelling. This effect has been discussed quantitatively in much detail in  for the case of water entrance by a heat treatment procedure. In the present Report we include also data by Shelby  obtained by in-situ generation of hydroxyl in gamma-irradiated silica. This procedure resulted in stronger density changes compared with the heat-treated material. Independent of this fact, we can show that the same volume swelling results for both methods. As an application of the results an estimation of the molar volume of hydroxyl is made.