Laboratory experiments were carried out with different types of natural and artificial seawater to study the aqueous degradation kinetics of the photolabile compounds CH2I2, CH2BrI, and CH2CII. Irradiation studies were carried out with a 1-kW Xe lamp, optically filtered to simulate the solar spectrum at the earth's surface. Halocarbon concentrations in the samples were analyzed by purge-and-trap gas chromatography/mass spectrometry. Generally, the compounds studied were found to follow first-order removal kinetics on irradiation. However, in the case of CH2I2, deviations from first-order removal occurred after a few minutes of irradiation, probably indicating radical recombination. Photolytic lifetimes varied from 12 min for CH2I2 to 13 h for CH2CII in natural surface seawater at 15 °C and an irradiation intensity corresponding to overhead sun (solar zenith angle = 0°). Photolysis of CH2I2 in artificial and natural seawater generated CH2CII with a yield of 25-30%, suggesting thatthis reaction is an important source of marine CH2CII. Dark-incubations of CH2I2 for up to one week showed that this compound does not undergo nucleophilic attack by chloride, indicating that photolysis is the main abiotic degradation mechanism of CH2I2 in seawater.