The molar heat capacities of La₂Mo₂0₉ and La_1.9Sr_0.1MO₂0_9-δ were obtained using the differential scanning calorimetry (DSC) technique in a temperature range from 298 to 1473 K. The DSC curve of La₂Mo₂0₉ showed an endothermal peak around 834 K corresponding to a first-order monoclinic-cubic phase transition, and the enthalpy change accompanying this phase transition is 5.99 kJ/mol. No evident endothermal peak existed in the DSC curve of La_1.9Sr_0.1MO₂0_9-δ, but a broad thermal anomaly existed in its heat capacity curve at around 832 K. In addition, the heat capacity values of La₂Mo₂0₉ and La_1.9Sr_0.1MO₂0_9-δ began to decrease at 1196 and 1330 K, respectively. The non-transitional heat capacity values of La₂Mo₂0₉ and La_1.9Sr_0.1MO₂0_9-δ were formulated using multiple regression analysis in two temperature ranges.