AbstractSicilian lemon essential oil is a very high quality product which is offered to the international market at prices that are higher than those of the lemon essential oils produced in other citrus‐growing countries.Market competition sometimes leads producers to ‘blend’ genuine essential oil with less expensive products, which are generally ‘citral’ and orange terpenes.These terpenes, which are usually obtained from orange essential oil produced abroad, possess characteristics which modify the composition of lemon essential oil, even if added in only moderate quantities.In particular, the percentage of Δ3‐carene and the ratios of Δ3‐carene/α‐terpinene and Δ3‐carene/camphene can provide a good indication of the possible addition of orange essential oil or ‐of by‐products obtained therefrom to genuine lemon essential oil.After a study made on a great number of genuine lemon essential oils, on several samples of sweet orange terpenes, and on a large number of lemon oil and orange terpene blends we propose a gas chromatographic method which allows the detection of the addition of even 5% of sweet orange oil terpenes to gen