The water vapor absorption cross section was measured at iodine laser generation frequency (n0=7603.14 cm-1), which was determined to be (1.15 ± 0.15)Ч10-24cm2 at 1 aim air pressure. The technique for opto-acoustic measurements of small absorption coefficient values of gas, having active adsorption on the cell walls, has been developed. The calculated value of the atmospheric-air absorption coefficient under normal conditions was fully determined by water vanor radiation absorption and was ig3Ч10-7см-1.