A carrier oil is designed to dilute your favorite essential oil so that it can topically applied on your skin. This type of oil serves as the base that carries the essential oil that is being mixed into it. In many cases, the amount of carrier oil is usually more than the essential oil.
Most cold pressed carrier oils used in aromatherapy typically have a shelf life of between 9 and 15 months, again depending on the particular oil in question and how well it is stored. Grapeseed oil has perhaps one of the shortest shelf lives at around 6 to 9 months, with Borage, Carrot and Evening Primrose oil close behind at around 10 to 12 months. These oils in particular are susceptible to damage caused by temperature changes.
At the other extreme, Coconut and Jojoba oil (which is really a liquid wax) will keep for many, many years. The Quinessence 5% and 10% Dilution Range uses Jojoba oil as a diluent for this very reason – these products will keep for many years as opposed to a dilution produced with almond or grapeseed oil which would only last for a few months.
Carrier oils will go rancid eventually due to the presence of unsaturated fatty acids which tend to oxidize quickly if not stored properly. However, this process usually takes up to a year and these products should have been used long before such a long period. This is one very good reason to buy smaller sizes of these products more regularly, rather than the one purchase of a larger size. If you are not going to use the oil quickly it can be a false economy.