In adiabatic low-pressure and dynamic calorimeters the temperature dependence of the standard molar heat capacity C-p,m(o) of dibenzo-p-dioxin and 1,2,3,4-tetrachlorodibenzo-p-dioxin have been determined at temperatures in the range T = 5 K to T = 490 K: from T = 5 K to T = 340 K with an accuracy of about 0.2 per cent and with an accuracy of 0.5 per cent to 1.5 per cent between T = 340 K and T = 490 K. The temperatures, enthalpies, and entropies of melting of the above compounds have been determined. The experimental data were used to calculate the thermodynamic functions C-p,m(o)/R, Delta H-T(o)m(o)/(R . K) Delta S-T(o)m(o)/R, and Phi (o)(m) = Delta S-T(o)m(o) - Delta H-T(o)m(o)/T (where R is the universal gas constant) in the range T --> 0 to T = 490 K. The isochoric heat capacity C-V,C-m of both dioxins has been estimated over the range T --> 0 to T-fus. The effect of substitution of four hydrogen atoms by chlorine atoms on the lattice and atomic components of the isochoric heat capacity was considered.