Absorption cross sections for oxygen and water vapor were measured using a low-pressure mercury lamp optically filtered to isolate the emission close to 184.9 nm. The cross-sections were determined for conditions typically used in the calibration of OH, HO2 and RO2 field instruments that employ photolysis of water vapor as a radical source, and O2 absorption as the actinometer for lamp flux. For water vapor, an absorption cross-section of 7.22 (¿0.22)¿10-20 cm2 molecule-1 was determined at 25 ¿C, in excellent agreement with recent studies, but ~30% higher than the recommended value. The absorption cross-section for O2 was found to vary with the O2 column and the choice and operating current of the lamp, reinforcing the requirement that practitioners of OH, HO2 and RO2 field measurements who calibrate by this method should measure effective O2 cross-sections at frequent intervals under their particular field conditions. ¿ 2000 American Geophysical Union