The molecular absorption spectra of hypochlorous, hypobromous, and hypoiodous acids have been studied by multiconfiguration self-consistent field (MCSCF)calculations with linear and quadratic response techniques. The complete form of the spin-orbit coupling (SOC) operator is accounted. The singlet-triplet transition to the lowest triplet state (3)A" <-- X(1)A', is shown to be responsible for the weak. long-wavelength tail absorption and photodissociation in these molecules. The transition is polarized along the O-X bond (X = Cl, Br, I) and has an oscillator strength equal 6 x 10(-6), 8 x 10(-5), and 2 x 10(-4) for hypoclorous, hypobromous, and hypoiodous acids, respectively. The second singlet-triplet transition (3)A' <--X(1)A' comes to the region of the first singlet-singlet (1)A" <-- X(1)A' absorption and contributes significantly to the total cross-section at wavelengths lambda approximate to 300-320 nm (X = Cl), lambda approximate to 340-360 nm (X = Br), and lambda approximate to 400 nm (X = I). In the last case the singlet-triplet transition (3)A' <-- X(1)A' produces predominant contribution to HOI absorption in the visible region. All states are dissociative, So the singlet-triplet absorption contributes to the yield of photolysis in the important near-UV and visible region close to the intense solar actinic flux. Contributions to the removal mechanisms for atmospheric HOCl, HOBr,and HOI species are shortly discussed. The minor loss process of ozone in troposphere because of the HOI reservoir sink is getting evident on the ground of this calculations. The importance of SOC accounting for atmospheric photochemistry problems is stressed.