A comprehensive study of thermodynamic properties of 1,1'-biadamantane in different phase states has been carried out. The heat capacity of the compound in the condensed state was measured in two adiabatic calorimeters in the temperature range from 5 to 370 K and in a single-cup scanning calorimeter from 315 to 6 10 K. Two solid-to-solid phase transitions at 336.3 +/- 0.3 K with = 1.154 +/- 0.019 kJ mol(-1) and 509.6 +/- 0.5 K with Delta H-trs(m)degrees = 1.30 +/- 0.03 kJ mol(-1), and fusion at T-fus similar to 561 K were revealed in this temperature interval. It was found that 1,1'-biadamantane did not form plastic crystals. The thermodynamic functions of the compound in the crystalline state were derived. The saturated vapour pressure of 1,1'-biadamantane from 393 to 443 K was determined by the Knudsen effusion method. In (P/Pa) = (32.82 +/- 0.37) - (13126 +/- 153) (K/T). The sublimation enthalpy was obtained from the results of the effusion measurements: Delta H-sub(m)degrees (417.8 K) = 109.1 +/- 1.3 kJ mol(-1), Delta H-sub(m)degrees (298.15 K) = 113.8 +/- 1.4 kJ mol(-1). The complete set of fundamentals was compiled from experimental spectral data and results of calculations in terms of the density functional theory (B3LYP/6-31G*). The combustion enthalpy of C20H30, Delta H-c(m)degrees (298.15 K, cr) = -(11801.3 +/- 7.3) kJ mol(-1), was determined by the bomb-calorimetry method and the enthalpy of formation, Delta H-f(m)degrees (298.15 K, cr) = -(356.4 +/- 7.8) kJ mol(-1), was obtained. The thermodynamic properties of 1, 1'-biadamantane in the ideal-gas state were calculated. (c) 2007 Published by Elsevier B.V.