The vaporization enthalpies {Delta(1)(g)H(m) (298.15 K)} of endo- and exo-tetrahydrodicyclopentadiene (THDCPD) have been measured by correlation gas chromatography. Values of (50.2 +/-2.3) kJ.mol(-1) and (49.1 +/- 2.3) kJ.mol(-1) have been obtained for the endo and exo isomers, respectively. The sublimation enthalpy of the endo isomer {Delta(cr)(g)H(m) (298.15 K) = (51.2 +/- 2.4) kJ.mol(-1)} has been obtained by combining fusion and vaporization enthalpies adjusted as necessary to T = 298.15 K. Low-temperature d.s.c. studies of both isomers reveal solid-to-solid phase transitions. The endo isomer, which is a plastic crystal at room temperature, exhibits a solid-to-solid phase transition at T = 214 K {Delta(cr(2))(cr(1))H(m)(214K) = (10.7 +/- 0.13) kJ.mol(-1)} and a melting temperature of 356.8 K {Delta(cr(1))(1)H(m)(356.8 K) = (3.48 +/- 0.2) kJ.mol(-1)} with some evidence of polymorphism. The exo isomer exhibits a solid phase transition at T = 162.1 K {Delta(cr(2))(cr(1))H(m) (162.1 K) = (3.18 +/-0.11) kJ.mol(-1)} and a melting temperature of 183.2 K {Delta(cr(1))(1)H(m) (183.2 K) = (1.20 +/- 0.04) kJ.mol(-1)}. Enthalpies of vaporization and sublimation are combined with the respective enthalpies of formation previously reported to yield the following gas-phase values: for endo THDCPD, Delta(f)H(m)degrees = -(61.9 +/- 3.2) kJ.mol(-1); and for