Absorption cross sections for water vapor at 184.9 nm were measured using a standard low-pressure mercury lamp light source, optically filtered to isolate the spectral region near the emission line. The light from the source was detected using a solar-blind phototube. Experiments were performed over a wide range of water column amounts, using neat water vapor and water vapor/nitrogen mixtures, with four methods to determine the water vapor concentration in the cell. Absorption cross sections were measured between 0 and 80 ¿C. The results from these experiments yield a cross section of 7.14(¿0.2)¿10-20 cm2 molecule-1at 25 ¿C with a small positive temperature dependence (about 4% between 0 and 80 ¿C). This result is about 30% greater than current recommendations. The absorption spectrum was also measured at discrete wavelengths between 183 and 193 nm using a scanning double monochromator system. The cross section of deuterated water vapor was determined at 184.9 nm. The implications of the changes in the H2O cross section for the calibration of atmospheric OH and HO2 measurements using the 184.9 nm photolysis of water vapor are discussed.¿ 1997 American Geophysical Union