A Hydrosol is basically the water fraction or portion of an essential oil distillation. They are also referred to as Hydrolats. During the distillation process, the aromatic compounds of essential oils are separated from the rest of the plant components and the steam used to remove them reconstitutes into a Hydrosol.  How much essential oil makes its way into a hydrosol is highly dependent upon the water solubility of the plant matter used and the parameters used in the distillation process.  Distillation is truly an art where expertise in pressure, temperature and timing are all critical.