The essential oil has been studied of leaves of Senecio coincyi Rouy, an endemic species of Spain restricted to a very small area of the Central Iberian Peninsula. Samples from five locations were obtained by hydrodistillation and extraction and analysed by gas chromatography-mass spectrometry. The main compound was 1-tridecene (28.1 +/- 8.5%). The presence of unsaturated hydrocarbons (1-undecene, 1-dodecene and 1-tridecene) seems to indicate a chemotaxonomic relationship between Senecio coincyi and S. congestus.