Compounds in the La 2- xSr xCuO 4- y series have been synthesized in the single phase range 0 ≤ x ≤ 1. High temperature solution calorimetry in molten lead borate was used to determine the heats of formation from oxide components. The enthalpy of formation becomes more exothermic with increasing Sr substitution 0 ≤ x ≤ 0.15 and then remains approximately constant for 0.15 ≤ x ≤ 1. This trend can be explained by the balance between the (formal) substitution La 3+ + Cu 2+ = Sr 2+ + Cu 3+ and the loss of oxygen, keeping copper formally divalent but creating oxygen vacancies. Using a transposed temperature drop method, the heat of oxidation was determined to be -129 kJ/(mole O 2) by a comparison of samples with the same strontium content but a different oxygen content.