A release of hydrogen fluoride (HF), hydrogen iodide (HI) and water vapour occurred in a building at a chemical plant during cleaning of a spherical storage tank containing iodine pentafluoride (IF5), which is a basic chemical substance used to synthesise an intermediate compound in manufacturing seals. A portion of this pink-coloured cloud (characteristic of the presence of iodine) escaped to the outside via the workshop vents. Plugging the tank, relocating it to a confined room and starting up the room’s lowering ramp all served to limit the duration of this release to just 1 hour. No environmental, human or operational consequences were identified. According to the plant operator, less than 40 kg of HF and HI were actually discharged. The aqueous effluents were channelled into the backup basin. The accident took place on a storage tank scheduled to be dismantled that had previously been emptied (the remaining 30 kg of IF5 corresponded to the volume between the dip tube and the tank bottom) and was supposed to be cleaned by spraying water. An obstruction of the hose exiting the tank caused content pressure to rise, given the decomposition of IF5 into HF and HI; afterwards, the hose broke leading to tank decompression inside the storage room. This washing step would be eliminated from the site as a result of definitive production shutdown. Nonetheless, personnel were still reminded of the need to attentively study all procedures associated with uncommon operations prior to their implementation.