Values for ΔfH°(298.15 K), S°(298.15 K), and Cp°(T) (5⩽T/K⩽6000) are computed by density functional B3LYP/6-31G(d,p) and B3LYP/6-311+G(3df,2p) calculation methods for 12 monochloro and dichloro dibenzo-p-dioxins and dibenzofurans: 1-chloro dibenzo-p-dioxin, 2-chloro dibenzo-p-dioxin, 1,6-dichloro dibenzo-p-dioxin, 1,8-dichloro dibenzo-p-dioxin, 1,9-dichloro dibenzo-p-dioxin, 2,8-dichloro dibenzo-p-dioxin, 3-chloro dibenzofuran, 4-chloro dibenzofuran, 1,6-dichloro dibenzofuran, 3,6-dichloro dibenzofuran, 3,7-dichloro dibenzofuran, and 4,6-dichloro dibenzofuran. Molecular structures and vibration frequencies are determined at the B3LYP/6-31G(d,p) level of theory. Isodesmic reactions are utilized at each calculation level to determine the enthalpy of formation of each species. Contributions to the entropy and the heat capacity from translation, vibration, and external rotations are calculated using the rigid-rotor-harmonic-oscillator approximation based on the B3LYP/6-31G(d,p) structures. The enthalpies of formation for 1,4-dioxin, furan, 2,3-benzodioxin, 2,3-benzofuran, dibenzo-p-dioxin, and dibenzofuran are also calculated. Thermochemical properties of two composite central atom groups and four interaction groups are derived for use in a group additivity scheme to calculate these thermochemical properties of polychlorinated dibenzo-p-dioxins and dibenzofurans.