In this work, laboratory chamber experiments of gas-phase methyl iodide photolysis in the presence of ozone at three relative humidity conditions were performed to study the formation and physico-chemical properties of iodine oxide particles. The obtained results revealed significant morphological changes of iodine oxide particles that were observed to depend on relative humidity. The formed iodine oxide particles under dry condition were supposed to be agglomerates of fine hygroscopic crystals. On the other hand, a humid atmosphere was observed to favor the formation of isomeric, tetragonal and orthorhombic hygroscopic crystals potentially composed of HIO 3 likely formed from progressive hydration of iodine oxide clusters. This process leads to a release of molecular iodine, I 2 , which may indicate a potential role of I 2 O 4 in the particles' evolution processes. The obtained results on the iodine oxides' behavior are important to the nuclear power plant safety industry since many of the organic iodides that may be released during a major nuclear power-plant accident contain radioactive isotopes of iodine that are known to have lethal or toxic impacts on human health.