The thermodynamic properties of the lanthanum manganites, La1-xMn1-yO3-z, have been reevaluated within the ideal association model to simultaneously account for excess oxygen and lanthanum nonstoichiometry of LaMnO3+delta, La0.95MnO3+delta and La0.9MnO3+delta. It has been found that a new set of parameters can reproduce the essential features of the thermogravimetric results reported in literature; that is, the averaged manganese Valence at P(O-2) = 1 atm does not depend largely on the lanthanum nonstoichiometry between LaMnO3+delta and La0.9MnO3+delta. Beyond this composition,however, the thermodynamic model has predicted that the averaged manganese valence increases with decreasing lanthanum stoichiometry and has confirmed the key idea for explaining the chemical reactivity of the lanthanum manganite such as the La2Zr2O7 formation and the Mn3O4 precipitation.