Polychlorinated dibenzo-p-dioxins (PCDDs) are very harmful and toxic environmental pollutants. PCDDs emitted from various sources are chemically transformed and decomposed through reactions with the OH radicals in the atmosphere. The reaction mechanism for PCDD decomposition has not been understood completely because they are difficult to deal with due to their toxicities and very low vapor pressure. Therefore, we have carried out density functional theory calculations using the B3LYP/6-31G** method for the initial OH radical reactions with four D-2h symmetric congeners, namely, dibenzo-p-dioxin (DD), 1,4,6,9-tetrachlorodibenzo-p-dioxin (1469-TCDD), 2,3,7,8-tetrachlorodibenzo-p-dioxin (2378-TCDD), and octachlorodibenzo-p-dioxin (OCDD) as reference compounds among 75 PCDDs to locate favorable reaction sites and reaction pathways for the first time. We found that the OH-substituted product (i.e., hydroxylated PCDD) is thermodynamically more stable than the PCDD-OH adduct. But the transition states for substitutions are higher than additions. The relative energies of the adducts and their transition states for the OH radical addition have negative values in terms of DeltaE(0) in agreement with experimentally measured activation energy of DD. The OH radical addition to D-2h congeners can attack three different carbon sites, viz., oxygen-bonded carbon (C-gamma), alpha-position carbon (C-alpha), and beta-position carbon (C-beta). Among three adduct isomers, the gamma adduct, formed by the OH radical addition at an ether linkage carbon, is energetically more stable than the alpha or beta adducts. The first step for the OH radical addition into dioxin shows no significant dependence with the chlorination pattern. There is not much difference between 1469-TCDD and 2378-TCDD for the OH addition reaction. The geometrical parameters of the OH adduct show a marked change around the head carbon. But the structural parameters of the other unattacked benzene ring are nearly conserved after the OH radical addition.