A study has been made of the drying of wool in vacuo. The time required to reach the equilibrium weight decreases with increasing temperature of drying, and the equilibrium weight decreases with the increasing temperature up to 140 degrees C. The equilibrium weight obtained in vacuo is less than the weight obtained in air under the same conditions. The absolute dry weight of a wool sample is defined as that weight obtained on heating the sample in vacuo to 140 degrees C for at least 10 min. The activation energy of drying increased markedly at low regains. From comparison with partially acetylated wool it was concluded that drying wool at low regains involves the removal of water from hydrophilic sites.