The wavelength-dependence of CH₂I₂ photolysis was studied in the laboratory using natural seawater. Irradiations were carried out using a 1 kW Xe lamp equipped with a monochromator, and halocarbon concentrations were determined by purge-and-trap gas chromatography/mass spectrometry. The absolute quantum yields for CH₂I₂ photodissociation were determined between 290 and 365 nm, and varied between 0.27 and 0.39. Our calculations show that photolysis of CH₂I₂ under natural sunlight conditions occurs predominantly between 300 and 350 nm. In seawater, calculated CH₂I₂ photolysis rates decrease quickly with depth, with 90% attenuation occurring in the top 10–25 m depending on the water clarity. A simple photochemical box model of a shallow equatorial mixed layer showed that most of the CH₂I₂ is photolysed before reaching the sea surface, where this compound is depleted with respect to the underlying water column.