Values for calculating the enthalpies of formation and standard entropies of chlorinated dibenzo-p-dioxins and dibenzofurans are obtained. The enthalpy of formation for dibenzo-p-dioxin derived from values for related compounds is -55 kJ mol(-1). The value of Delta(f)H for dibenzofuran obtained by Chirico et al., 55.2 kJ mol(-1), is recommended. Basic reductions in the enthalpy of formation of 29.5 kJ mol(-1) for each chlorine substitution, with positive corrections of 7.5 kJ mol(-1) for each ortho- and 2.5 kJ mol(-1) for each para-substitution, are derived from the enthaplies of formation of chlorinated benzenes. It is noted that a recent evaluation of the enthalpies of formation acid sublimation of the mono- and dichlorophenols indicates that the enthalpies of formation of these and possibly other chlorinated aromatic compounds vary more regularly with degree of chlorination than previously thought, and the model of chlorination based on phenols due to Shaub is thus, in general, unsatisfactory. A previous model of dibenzo-p-dioxin and dibenzofuran chlorination based on benzoquinone chlorination due to the author may underestimate the reduction in enthalpy of formation for the higher degrees of chlorination. The values 388 J mol(-1) K-1 (Shaub) and 374.4 J mol(-1) K-1 (Chirico et al.) for the standard entropies of dibenzo-p-dioxin and dibenzofuran are recommended, with each chlorination increasing the entropy by 22.6 J mol(-1) K-1.