The standard molar enthalpy of combustion of crystalline 2,3-dichlorodibenzo-p-dioxin was determined by rotating-bomb calorimetry : Delta(c)H(m) degrees = -(5406.4 +/- 6.6)kJ.mol(-1). Using this result and the standard molar enthalpy of sublimation, Delta(sub)H(m) degrees = (108.6 +/- 1.0)kJ.mol(-1) the values of the standard molar enthalpies of formation of 2,3-dichlorodibenzo-p-dioxin in the crystalline and gaseous states were found to be -(220.5 +/- 6.8)kJ.mol(-1) and -(111.9 +/- 6.9)kJ.mol(-1), respectively. The standard molar enthalpy of formation of gaseous 2,3,7,8-tetrachlorodibenzo-p-dioxin was estimated : Delta(f)H(m) degrees(g) approximate to -164.6 kJ.mol(-1).