The occurrence of oxygen heterocyclic compounds in the peel oils from differentCitrusspecies has been investigated by high performance liquid chromatography. The use of a photodiode array detector has facilitated the identification and quantification of the individual coumarins, psoralens and polymethoxyflavones present. Certain oxygen heterocyclic compounds are unique to a particular species, but the majority occur widely in the peel oils. Nevertheless, the patterns of occurrence are sufficiently diverse to permit the detection of peel oil from one species in that from another.