Oxygen nonstoichiometry (delta) of "undoped", polycrystalline BaTiO3-delta (but with two different nominal purity) has been measured as a function of oxygen partial pressure in the range of 10(-15) less than or equal to P-O2/atm less than or equal to 0.1 at elevated temperatures (1073 less than or equal to T/K less than or equal to 1273) by a solid-state coulometric titration technique. Each isotherm varies sine-hyperbolically against log(P-O2) with an inflexion point therein (corresponding to the electronic stoichiometric composition or delta = 0). The nonstoichiometry variation is attributed to oxygen vacancies compensated mostly by background and/or intrinsic acceptors in the higher P-O2 region and by electrons in the lower P-O2 region. The purity effect on nonstoichiometry is discussed. The intrinsic electronic excitation equilibrium constant is evaluated and compared exhaustively with the reported. The relative partial molar enthalpy and entropy of component oxygen are also evaluated as functions of the nonstoichiometry.