Abstract The properties of Bi-, Tl-cuprates and YBa 2Cu 3O 7 − δ ceramics were studied after annealings with gaseous iodine. Magnetic measurements show significant enhancement and stabilization of the high- T c phase (2:2:2:3) fraction in iodinated Bi- and Tl-cuprates. Simultaneous use of Raman spectroscopy, analytical transmission electron microscopy (ATEM) with electron energy loss (EEL), and X-ray photoelectron (XPS) spectroscopy in conjunction with energy-dispersive X-ray analysis (EDX) and X-ray diffraction (XRD) show the presence of iodine in the lattice structure. On the one hand, Raman and XPS techniques provide evidence for the formation of I - 3 and possibly I - 5 in these systems. On the other hand, EEL experiments carried out on iodinated Bi-cuprates and KI and K +I - 3 crystalline standards provide evidence for I - ions in the structure. To explain the two sets of experiments we propose that polyiodides can be present in the host material with I - 3 and I - 5 preferentially located at grain boundaries or crystal defects. I - would be intercalated in the host lattice.