Adiabatic calorimetry provides accurate and precise specific heat (Cp) data. From this data, thermodynamic functions may be calculated. Cu2OCl2, melanothallite, became of interest as part of a study of a particular thermochemical cycle. The experimental specific heat data and the calculated thermodynamic functions are reported here. Free energies of formation, calculated from the thermodynamic functions, suggest the particular cycle of interest with this compound as an intermediate is not feasible; uncertainty as to the accuracy of CuO and CuCl2 data used in the calculations indicate further study may be necessary. Upon collection of the specific heat data, an antiferromagnetic transition was observed at 70 K; this led to examination of the magnetic heat capacity and entropy of the transition in melanothallite. The entropy of the transition was estimated to be 18.1 % and 7.5 % of 2Rln2 by two methods. A theoretical calculation using an Ising model produced a result of 39 %. This is consistently low when compared to the entropies of the antiferromagnetic transitions of CuO and CuCl2. This suggests geometric frustration. This thesis reports the thermodynamic functions calculated from the specific heat; the examination of the magnetic entropy; and repairs to an adiabatic apparatus involved in the collection of this data.