Biochemical and morphological components of 16 Pelargonium species and the P. ×hortorum interspecific complex were examined. Inflorescences and leaves of each species were analyzed for anacardic acids and the presence of glandular trichomes. Three species of the section Ciconium, P. acetosum, P. frutetorum, and P. inquinans, produced anacardic acids in association with glandular trichomes. only P. inquinans and P. frutetorum contained ω5- anacardic acids. An evolutionary model for the origin of anacardic acids and ω5- desaturation is proposed.