The essential oils of Pelargonium sidoides DC. and P. reniforme Curt. were obtained by hydrodistillation from the leaves of the plants in 0.52% and 0.71% yields, respectively, related to the dry weight. Their composition was analysed by GLC and GC±MS. About 230 components have been detected in each of the Pelargonium oils, of which 102 (P. sidoides) and 81 (P. reniforme) could be unambiguously identi®ed, accounting for about 65% and 49% of the total peak area, respectively. For both species, sesquiterpenes (approximately 60%) were the dominating components, with caryophyllene (2.3%) and caryophyllene epoxide (13%) as the most abundant compounds of the sesquiterpene fraction of the oil of P. sidoides, and the sesquiterpene hydrocarbons d-selinene (4.2%) and d-cadinene (4.0%) as the main representatives of the oil of P. reniforme. Other major groups of the oil of P. sidoides comprised monoterpenes (16%) and phenylpropanoids (9%), the major compounds of the latter group being methyleugenol (4.3%) and elemicin (3.6%). By contrast, only monoterpenes were found in reasonable amounts (4.7%) in the related species P. reniforme. Detection of anacardic acids and furan-type constituents provide a rational explanation for insect deterrency by Pelargonium species.