Tea tree oil, also known as melaleuca oil or ti tree oil, is an essential oil with a fresh camphoraceous odor and a colour that ranges from pale yellow to nearly colourless and clear. It is derived from the leaves of the tea tree, Melaleuca alternifolia, native to Southeast Queensland and the Northeast coast of New South Wales, Australia.

The essential oil of the lemon-scented gum mainly consists of citronellal (80%), produced largely in Brazil and China. Unrefined oil from the lemon eucalyptus tree is used in perfumery, and a refined form of this oil is used in insect repellents, especially against mosquitoes. The refined oil’s citronellal content is turned into cis- and trans- isomers of p-menthane-3,8-diol (PMD), a process which occurs naturally as the eucalyptus leaves age. This refined oil, which includes related compounds from the essential corymbia citriodora.