The hygroscopicity of poly-alpha-olefin based oils is studied. Moisture saturation of all oils used in this study is reached by 60 h into the tests. The dependence of the hygroscopocity of the oils on the molecular mass of the oligomer is determined. The presence of functional additives in the oils does not greatly affect their hygroscopicity, while polar compounds (products of aging) sharply (almost twofold) increase the hygroscopicity.