the usefulness of the GC-MS information on the composition of essential oils varies from product to product. Where the principal ingredient is an important and obvious part of the oils action then standards can be applied to the oil for example, tea tree, eucalyptus and lavender. Some oils take their actions from the more minor constituents, or have their action modified by the minor constituents, so standards are less helpful in the case of these oils. As all the oils oxidise GC-MS testing can assist the nose in telling whether the oil is out of date. But the nose is the best guide!