A process for producing a tertiary olefin by decomposing a tertiary alkyl ether comprises a) decomposing at least one ether to a product containing an alcohol and a tertiary olefin, b) fractionating at least a portion of the product from a) in a fractionation zone (C1) to obtain the tertiary olefin and the alcohol, c) purifying at least a portion of the tertiary olefin obtained from step b) wherein said portion is sent to a water washing extraction zone (L1) from which a fraction (D) containing the tertiary olefin is recovered, and d) a step in which at least a portion of the fraction (D) from c) is sent to a separation zone (Co) from which a liquid aqueous fraction (Le) and a liquid hydrocarbon fraction (Lc) containing the major portion of the tertiary olefin are recovered.