The absorption spectrum of iodine dioxide (OIO), obtained at high resolution between 540 and 605 nm by cavity ring-down spectroscopy, exhibits no evidence of coarse rotational structure. Furthermore, a laser induced fluorescence spectrum was not observed when pumping the molecule in this region of the visible spectrum. Ab initio quantum calculations were performed on the ground and first excited states of OIO. The rotational envelopes of the observed absorption bands are very satisfactorily simulated if the lifetime of the excited state is 200 ± 50 fs, indicating prompt predissociation. Quantum calculations indicate photolysis to I + O₂, rather than O + IO. The estimated photodissociation rate of OIO in daylight ranges from 0.36 to 2.2 s⁻¹, depending on the choice of absolute cross-section, which explains why OIO has only been observed in the atmosphere after sunset. Photolysis to yield atomic I will enhance the O₃-depleting potential of iodine in the remote marine boundary layer.