The composition of the essential oils of Senecio vulgaris, S. jacobaea, S. congestus and S. fuchsii was investigated by means of GLC, CC and GC-MS. The principal constituents differ; S. congestus: 1-tridecene and 1-undecene, S. jacobaea: germacrene D and 1-undecene, S. vulgaris: β-caryophyllene, α-copaene and myrcene, S. fuchsii: anhydro-oplopanone and an unknown sesquiterpene C15H24. In all the oils anhydro-oplopanone is present. The spectra of this compound and of the synthesized anhydro-oplopanols are given.