The labelling of diiodofluorescein with131I using the ultraviolet waves as an excitation energy has been tried. Different normal alcohols have been used as solvents. The different parameters affecting the labelling yield such as, presence of water in different proportions, pH of the medium and presence of different concentrations of carrier, have been studied. Also, the effective lines of the Hg lamp have been elucidated. The labelling mechanism in different conditions has been explained. The study revealed that higher yields were obtained when using a mixture of ethanol and water as solvent at pH ∼ 5.