The determination is carried out by converting the carbonyl compounds to oximes by reaction with free hydroxylamine liberated in a mixture of hydroxylammoniumchloride and potassium hydroxide. The excess alkali is titrated with hydrochloric acid solution, either colorimetrically or potentiometrically. The method is applicable to essential oils which contain carbonyl compounds (especially ketones, excluding methylketones) which are difficult to convert to oximes by the method specified in ISO 1279. The method is not applicable to essential oils which contain substantial amounts of esters of other alkali-sensitive constituents.