Heat capacity and enthalpy measurements by using adiabatic calorimetry for temperatures between 6 K and 440 K are reported for bicyclohexyl (Chemical Abstracts registry number [92-51-3]). The triple point temperature and the enthalpy of fusion were determined. Two different phase-change sequences in the solid state were observed depending upon the thermal history of the sample. Discord between literature values for temperatures and enthalpies of phase transitions are shown to be due to hysteresis effects. Entropies and enthalpies for the condensed phases relative to T --> 0 are reported. Results are compared with literature calorimetric and spectroscopic studies. The stability relationships between the known phases of crystalline bicyclohexyl are established. Planned publications in this series will include reports of measurements leading to the calculation of standard molar Gibbs energies of formation for bicyclohexyl and phenylcyclohexane, and a revised thermodynamic analysis of the (biphenyl + hydrogen) reaction network.