Two different high-temperature calorimetry methods were used to determine the enthalpy of reaction for the reduction of Co3O4 to CoO. This reaction is particularly interesting for assessing the thermodynamics of the first step of the electrochemical half-cell reaction for conversion anodes based on Co3O4 at very low discharge currents. The enthalpy of reaction for the reduction of Co3O4 to CoO was determined to be ΔrH°=205.14±2.48kJ/mol and ΔrH°=204.46±3.17kJ/mol from transposed temperature drop and high-temperature oxide melt drop solution calorimetry, respectively. Using this data, the enthalpy of reaction for the first step of the half-cell reaction for Co3O4 at very low discharge currents involving the formation of Li2O and CoO was calculated as ΔrH°=−393.60±3.67kJ/mol and ΔrH°=−394.28±4.16kJ/mol from the transposed temperature drop and the high-temperature oxide melt drop solution calorimetry, respectively.