Abstract This review covers chemical aspects of the iodine stress corrosion cracking (ISCC) pellet-cladding interaction (PCI) failures which have occurred in some reactors with UO 2 fuel in unlined Zircaloy tubes. The cracking mechanism is discussed in relation to chemical factors. Cesium and iodine are released as fission products and form CsI but its dissociation iodine pressure p(I) is many orders of magnitude greater than chemical thermodynamic calculations predict, due to radiolysis. This enhanced p(I) can form ZrI 4 in regions where Zr is exposed (not protected by surface scale) and a van Arkel vapour transport reaction then causes rapid Zr transport from localised cladding areas, i.e. pitting, which later becomes a crack site. Requirements are a critical strain rate (which cracks protective scale), iodine pressure and time, i.e. a power ramp. Factors include that a stress caused by a power ramp may relax before the required gap chemistry is achieved and that a ramp also releases short-lived I and Cs fission products which add to the iodine pressure available.