The enthalpies of formation for gaseous dibenzo-p-dioxin and 75 polychlorinated dibenzo-p-dioxin congeners were calculated at the B3LYP density functional theory level using isodesmic reactions. Calculated values are compared with experimental data for dibenzo-p-dioxin, 1-chlorodibenzo-p-dioxin, 2-chlorodibenzo-p-dioxin, and 2,3-dichlorodibenzo-p-dioxin as well as with previous results from group additivity estimations and theoretical calculations. The calculated enthalpy of formation of dibenzo-p-dioxin, -52.8 +/- 3.0 kJ/mol, is in good agreement with the recent experimental value of -50.1 +/- 2.2 kJ/mol, whereas the discrepancies from experimental data for chlorinated dibenzo-p-dioxins range from 2 to 20 kJ/mol. The uncertainties of calculated enthalpies of formation are estimated to be from 3 to 30 kJ/mol.