Essential oils don't really go rancid. Unlike vegetable oils, essential oils are inherently their own natural preservatives .

However, although essential oils don't go rancid, they can oxidize and lose their effectiveness. Oxygen causes the molecules in the e.o. to break down and the oil can gradually lose its therapeutic properties. But this can take years. Citrus oils are the most prone to this. You can test your oils by its smell. As your oil oxidizes it will gradually lose its smell. If it smells weak, it has probably lost some of its effectiveness.

The best way to avoid this is to store your oils properly. keep caps tight. Store in cool, dark place (because sunlight and heat encourage oxidation). Doing this, your oils can last for years.

-Daphne