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Isoricinoleic acid (9-hydroxy-cis-12-octadecenoic acid; IR) is a potential renewable feedstock for the oleochemical industry, a precursor for the synthesis of antimicrobial compounds and a component of the seed oil of certain plants in the Apocynaceae. For a more detailed survey of this plant family, seeds of 18 species representing different subfamilies were obtained and acyl composition and oil content was determined. IR was observed only in species of the tribes Wrightieae and Nerieae in the Apocynoideae subfamily and is reported for the first time in the seed oil of the desert rose Adenium obesum in which it is present at a level of around 26 %. In contrast to previous reports, IR was not found in oil from Holarrhena species, H. antidysenterica and H. pubescens, nor in oil from Annona squamosa. To examine the oil structure, samples were analyzed using MALDI-TOF mass spectrometry. This technique proved to be a simple method to demonstrate the occurrence of the estolide 9-acetoxy-cis-12-octadecenoic acid in oil from Nerium oleander and gave further insight into the distribution of estolides within the oil, revealing the presence of tetra- and penta-acyl-TAG molecules, and molecules containing IR esterified to all three position of glycerol. For other species in which IR was observed, the HFA was found to be a component of seed TAG, but no secondary acylation of the hydroxyl groups was observed.