Doubly-labeled [U-14 C, 1,5 I] iodobenzene was synthesized, and its dilute solutions in hexane, benzene or methanol were stored at room or at low temperature to accumulate 1 ,S I decays. The 14 C-labeled products remaining after 1 "I had decayed were separated by gas chromatography and their yield determined radioanalytically. The fate of the highly excited [U-14 C]C,H S formed after the 155 I decay depends strongly on the matrix. In hexane aromatic rings decompose efficiently into fragments. In benzene they largely escape fragmentation, because of the ability of the surrounding ττ-electron molecules to facilitate intermolecular energy transfer. The results suggest that the charge built-up after the Auger transition and the energy deposited during neutralization, leading to super-excitation, is the main damaging event.