The formation of atmospherically relevant iodine oxides IxOy (x = 1,…,3, y = 1,…,7) has been studied experimentally using time-of-flight mass spectrometry combined with a soft ionisation source, complemented with ab initio electronic structure calculations of ionisation potentials and bond energies at a high level of theory presented in detail in the accompanying paper (Galvez et al., 2013). For the first time, direct experimental evidence of the I2Oy (y = 1,…,5) molecules in the gas phase has been obtained. These chemical species are observed alongside their precursors (IO and OIO) in experiments where large amounts of aerosol are also generated. The measured relative concentrations of the IxOy molecules and their dependence on ozone concentration have been investigated by using chemical modelling and rate theory calculations. It is concluded that I2O4 is the most plausible candidate to initiate nucleation, while the contribution of I2O5 in the initial steps is likely to be marginal. The absence of large I3Oy (y = 3,…,6) peaks in the mass spectra and the high stability of the I2O4-I2O4 dimer indicate that dimerisation of I2O4 is the key step in iodine oxide particle nucleation.