The standard molar energy of combustion of crystalline 2-chlorodibenzo-p-dioxin was determined by rotating-bomb calorimetry : Delta(c)U degrees(m) = -(5573.6 +/- 2.8) kJ . mol(-1). The standard molar enthalpy of sublimation was measured in a Calvet microcalorimeter : Delta(sub)H degrees(m) = (97.24 +/- 0.55) kJ . mol(-1). Using these results, the values of the standard molar enthalpies of formation of 2-chlorodibenzo-p-dioxin in the crystalline and gaseous states were found to be -(171.3 +/- 3.2) kJ . mol(-1) and -(74.1 +/- 3.3) kJ . mol(-1), respectively.