A method of removing the blue colour from an azulene-containing essential oil, comprising the addition of the oil to water buffered at pH 3-7.5 and its subsequent treatment with a laccase enzyme-mediator system, the laccase being selected from those derived from Trametes sp. and Myceliophthora thermophila in the presence of a mediator. Decoloured oils may be used in perfumery, where their colour was previously a major disadvantage.