Olive oil and its main constituent, oleic acid, have been labelled with125I using chloramine-T (CAT) as an oxidizing agent. The maximum yields of labelling in different organic solvents were found to vary between 10 to 50% for oleic acid. The values for olive oil were 70% of those of oleic acid. The labelled compounds were stable in allorganic solvents except for acetone where they start to decompose after 20 min of starting the experiments. The low yields could be attributed partly to a possible labelling of CAT or to the reaction of CAT with labelled compounds or their precursores.