The absorption cross sections for gaseous ClNO2 in the 200--370 nm region and for Cl2 in the 270--400 nm region have been determined at 298 K. The cross sections for Cl, are in excellent agreement with the literature. At λ<300 nm the absorption cross sections for ClNO2 are in good agreement with those of Illies and Takacs (1976/1977) and Nelson and Johnston (1981) but are higher than those of Martin and Gareis (1956) from 220 to 240 nm. In the tropospherically important region beyond 290 nm our results, which have been corrected for impurity Cl2 using a combination of mass spectrometry and UV absorption, fall between those of Martin and Gareis (1956) and Illies and Takacs (1976/1977) and are in excellent agreement with the values of Nelson and Johnston (1981). Estimates of the photolysis rate constants and photolytic lifetimes for ClNO2 for various solar zenith angles at the Earth's surface are reported. It is shown that atomic chlorine from ClNO2 photolysis may be a significant initiator of organic photooxidation in both moderately polluted and remote marine atmospheres. ¿ American Geophysical Union 1992