AbstractLong chain ω‐123‐I‐labelled fatty acids such as 16‐I‐hexadecanoic‐, 17‐I‐heptadecanoic‐, and 16‐I‐hexadec‐9‐enoic acid have been prepared without adding carrier by nucleophilic 123I‐for‐Br exchange in acetone, in the melt, and by use of phase transfer catalysts in non‐polar solvents. While preparation in refluxing acetone required reaction times of more than 2 hours to reach saturation yields, halogen exchange in molten bromofatty acids at temperatures of about 150 °C gave rise to yields of about 80% of radiochemically pure 17‐123I‐heptadecanoic acid within only 5 minutes. Comparable yields were also obtained using Kryptofix 221 in refluxing toluene after 10 minutes.