Steam distillation is a special type of distillation (a separation process) for temperature sensitive materials like natural aromatic compounds.
Many complex organic compounds tend to decompose at sustained high temperatures. Separation by normal distillation would then not be an option, so water or steam are introduced into the distillation apparatus. By adding water or steam the boiling point of the compounds is depressed (see article on Colligative properties), allowing them to evaporate at lower temperatures, preferably below the temperatures at which the deterioration of the material becomes appreciable. If the substances to be distilled are very sensitive to heat, steam distillation can also be combined with vacuum distillation. After distillation the vapors are condensed as usual, usually yielding a two-phase system of water and the organic compounds, allowing for simple separation.
Steam distillation is employed in the manufacture of ethereal oils for, for instance, perfumes. In this method steam is guided over the plant material containing the desired oils. It is also employed in the synthetic procedures of complex organic compounds.